diff --git a/src/main/java/city/makeour/moc/MocClient.java b/src/main/java/city/makeour/moc/MocClient.java
index e0d34af..6baedcc 100644
--- a/src/main/java/city/makeour/moc/MocClient.java
+++ b/src/main/java/city/makeour/moc/MocClient.java
@@ -2,12 +2,15 @@
 
 import java.security.InvalidKeyException;
 import java.security.NoSuchAlgorithmException;
+import java.util.Map;
 
+import org.springframework.web.client.RestClient;
 import org.springframework.web.client.RestClient.ResponseSpec;
 
 import city.makeour.moc.ngsiv2.Ngsiv2Client;
 import city.makeour.ngsi.v2.api.EntitiesApi;
 import city.makeour.ngsi.v2.invoker.ApiClient;
+import city.makeour.ngsi.v2.model.UpdateExistingEntityAttributesRequest;
 
 public class MocClient {
     protected Ngsiv2Client client;
@@ -146,4 +149,36 @@ public ResponseSpec getEntity(String entityId, String type) {
         return this.entities().retrieveEntityWithResponseSpec(entityId, type, null, null, "keyValues");
     }
 
+    public ResponseSpec updateEntity(String id, String type, Map<String, Object> attributesToUpdate) {
+        if (id == null || id.isBlank()) throw new IllegalArgumentException("id is required");
+        if (type == null || type.isBlank()) throw new IllegalArgumentException("type is required");
+        if (attributesToUpdate == null) attributesToUpdate = java.util.Collections.emptyMap();
+    
+        try {
+            // Existence check
+            this.entities()
+                .retrieveEntityWithResponseSpec(id, type, null, null, "keyValues")
+                .toEntity(Object.class);
+
+            // Exists -> POST (keyValues 形式でそのまま送る)
+            return this.client.updateEntityAttributes(
+                id,
+                "application/json",
+                attributesToUpdate, // Object(Map) をそのまま PATCH
+                type,
+                "keyValues"
+            );
+
+        } catch (org.springframework.web.client.RestClientResponseException e) {
+            if (e.getStatusCode().value() != 404) throw e;
+
+            // Not found -> create (従来通り)
+            java.util.Map<String, Object> body = new java.util.HashMap<>();
+            body.put("id", id);
+            body.put("type", type);
+            body.putAll(attributesToUpdate);
+            return this.createEntity("application/json", body);
+        }   
+    }
+
 }
diff --git a/src/main/java/city/makeour/moc/auth/srp/Helper.java b/src/main/java/city/makeour/moc/auth/srp/Helper.java
index 97884dd..1db5c7a 100644
--- a/src/main/java/city/makeour/moc/auth/srp/Helper.java
+++ b/src/main/java/city/makeour/moc/auth/srp/Helper.java
@@ -36,6 +36,12 @@ public static byte[] padHex(BigInteger bigInt) {
     }
 
     public static byte[] hexStringToByteArray(String s) {
+
+        // 奇数桁なら先頭に 0 を追加
+        if (s.length() % 2 != 0) {
+            s = "0" + s;
+        }
+
         int len = s.length();
         byte[] data = new byte[len / 2];
         for (int i = 0; i < len; i += 2) {
diff --git a/src/main/java/city/makeour/moc/ngsiv2/Ngsiv2Client.java b/src/main/java/city/makeour/moc/ngsiv2/Ngsiv2Client.java
index c9bb224..2cd025e 100644
--- a/src/main/java/city/makeour/moc/ngsiv2/Ngsiv2Client.java
+++ b/src/main/java/city/makeour/moc/ngsiv2/Ngsiv2Client.java
@@ -71,4 +71,71 @@ public RestClient.ResponseSpec createEntity(String contentType, Object body) {
                     localVarReturnType);
         }
     }
+
+    public RestClient.ResponseSpec updateEntityAttributes(
+        String entityId,
+        String contentType,
+        Object body,
+        String type,
+        String options
+    ) {
+        if (entityId == null) {
+            throw new RestClientResponseException(
+                "Missing the required parameter 'entityId' when calling updateEntityAttributes",
+                HttpStatus.BAD_REQUEST.value(), HttpStatus.BAD_REQUEST.getReasonPhrase(),
+                (HttpHeaders) null, (byte[]) null, (Charset) null
+            );
+        }
+        if (contentType == null) {
+            throw new RestClientResponseException(
+                "Missing the required parameter 'contentType' when calling updateEntityAttributes",
+                HttpStatus.BAD_REQUEST.value(), HttpStatus.BAD_REQUEST.getReasonPhrase(),
+                (HttpHeaders) null, (byte[]) null, (Charset) null
+            );
+        }
+        if (body == null) {
+            throw new RestClientResponseException(
+                "Missing the required parameter 'body' when calling updateEntityAttributes",
+                HttpStatus.BAD_REQUEST.value(), HttpStatus.BAD_REQUEST.getReasonPhrase(),
+                (HttpHeaders) null, (byte[]) null, (Charset) null
+            );
+        }
+
+        Map<String, Object> pathParams = new HashMap();
+        pathParams.put("entityId", entityId);
+
+        MultiValueMap<String, String> queryParams = new LinkedMultiValueMap();
+        HttpHeaders headerParams = new HttpHeaders();
+        MultiValueMap<String, String> cookieParams = new LinkedMultiValueMap();
+        MultiValueMap<String, Object> formParams = new LinkedMultiValueMap();
+
+        // ?type=... と ?options=...
+        queryParams.putAll(this.apiClient.parameterToMultiValueMap(null, "type", type));
+        queryParams.putAll(this.apiClient.parameterToMultiValueMap(null, "options", options));
+
+        headerParams.add("Content-Type", this.apiClient.parameterToString(contentType));
+
+        String[] localVarAccepts = new String[] { }; // 204 No Content 想定
+        List<MediaType> localVarAccept = this.apiClient.selectHeaderAccept(localVarAccepts);
+        String[] localVarContentTypes = new String[] { "application/json" };
+        MediaType localVarContentType = this.apiClient.selectHeaderContentType(localVarContentTypes);
+        String[] localVarAuthNames = new String[] { };
+
+        ParameterizedTypeReference<Void> localVarReturnType = new ParameterizedTypeReference<>() {};
+        return this.apiClient.invokeAPI(
+            "/v2/entities/{entityId}/attrs",
+            HttpMethod.POST,
+            pathParams,
+            queryParams,
+            body,  // ← Map<String,Object> 等をそのまま渡す
+            headerParams,
+            cookieParams,
+            formParams,
+            localVarAccept,
+            localVarContentType,
+            localVarAuthNames,
+            localVarReturnType
+        );
+    }
+
 }
diff --git a/src/test/java/city/makeour/moc/MocClientTest.java b/src/test/java/city/makeour/moc/MocClientTest.java
index acc9b68..20149b3 100644
--- a/src/test/java/city/makeour/moc/MocClientTest.java
+++ b/src/test/java/city/makeour/moc/MocClientTest.java
@@ -1,17 +1,21 @@
 package city.makeour.moc;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 
 import java.security.GeneralSecurityException;
 import java.security.NoSuchAlgorithmException;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 import java.util.UUID;
 
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.condition.EnabledIfEnvironmentVariable;
 import org.junit.jupiter.api.condition.EnabledIfEnvironmentVariables;
+import org.springframework.core.ParameterizedTypeReference;
 
 import city.makeour.ngsi.v2.api.EntitiesApi;
 import city.makeour.ngsi.v2.model.CreateEntityRequest;
@@ -145,5 +149,60 @@ void testCreateAndGetEntity_Minimal() throws GeneralSecurityException, NoSuchAlg
 
         assertNotNull(retrievedEntity);
         assertEquals(entityId, retrievedEntity.getId());
+    }    
+
+    @Test
+        @DisplayName("updateEntityのUpsert（作成・更新）ロジックをテストする")
+        @EnabledIfEnvironmentVariables({
+            @EnabledIfEnvironmentVariable(named = "TEST_COGNITO_USER_POOL_ID", matches = ".*"),
+            @EnabledIfEnvironmentVariable(named = "TEST_COGNITO_CLIENT_ID", matches = ".*"),
+            @EnabledIfEnvironmentVariable(named = "TEST_COGNITO_USERNAME", matches = ".*"),
+            @EnabledIfEnvironmentVariable(named = "TEST_COGNITO_PASSWORD", matches = ".*")
+        })
+    void testUpdateEntity_UpsertLogic() throws GeneralSecurityException, NoSuchAlgorithmException {
+        MocClient client = new MocClient();
+        client.setMocAuthInfo(System.getenv("TEST_COGNITO_USER_POOL_ID"), System.getenv("TEST_COGNITO_CLIENT_ID"));
+        client.login(System.getenv("TEST_COGNITO_USERNAME"), System.getenv("TEST_COGNITO_PASSWORD"));
+    
+        String entityId = "urn:ngsi-ld:TestUpsert:" + UUID.randomUUID().toString();
+        String entityType = "TestUpsertType";
+    
+        // 1. "作成" (Insert) pathのテスト
+        // エンティティが存在しない --> catchブロックの this.createEntity
+        Map<String, Object> initialAttrs = new HashMap<>();
+        initialAttrs.put("temperature", 25);
+        initialAttrs.put("humidity", 50); // "temperature" 以外の属性も指定
+    
+        client.updateEntity(entityId, entityType, initialAttrs);
+    
+        // 検証 (作成)
+        ParameterizedTypeReference<Map<String, Object>> mapType = new ParameterizedTypeReference<>() {};
+        Map<String, Object> createdEntity = client.getEntity(entityId, entityType).body(mapType);
+    
+        assertNotNull(createdEntity);
+        assertEquals(entityId, createdEntity.get("id"));
+        assertEquals(25, createdEntity.get("temperature"));
+        assertEquals(50, createdEntity.get("humidity"));
+    
+        // 2. "更新" (Update/PATCH) pathのテスト
+        // エンティティが既に存在する --> tryブロックの updateExistingEntityAttributesWithResponseSpec
+        Map<String, Object> updateAttrs = new HashMap<>();
+        updateAttrs.put("temperature", 30);      // 更新
+        updateAttrs.put("seatNumber", 10);       // 追加
+        updateAttrs.put("status", "active");
+    
+        client.updateEntity(entityId, entityType, updateAttrs);
+    
+        // 検証 (更新)
+        Map<String, Object> updatedEntity = client.getEntity(entityId, entityType).body(mapType);
+    
+        assertNotNull(updatedEntity);
+        // 更新・追加されている
+        assertEquals(30, updatedEntity.get("temperature"));
+        assertEquals(10, updatedEntity.get("seatNumber"));
+        // 最初の作成時から変更されず残っている
+        assertEquals(50, updatedEntity.get("humidity"));
+        
+        assertEquals("active", updatedEntity.get("status"));
     }
 }