diff --git a/class.png b/class.png
index c1df006..05af90d 100644
--- a/class.png
+++ b/class.png
Binary files differ
diff --git a/class.pu b/class.pu
index f8e5a03..672326e 100644
--- a/class.pu
+++ b/class.pu
@@ -41,6 +41,17 @@
   - active : boolean
 }
 
+class UniqueLoginValidator<UniqueLogin> <<ConstraintValidator>> {
+  - userRepository : SiteUserRepository
+  + isValid(String, ConstraintValidatorContext) : boolean
+}
+
+class UniqueLogin <<Constraint>> {
+  message() : String
+  groups() : Class<?>[]
+  payload() : Class<? extends Payload>[]
+}
+
 interface SiteUserRepository <<JpaRepository>> {
 	findByUsername(String)
 	existsByUsername(String)
@@ -48,6 +59,8 @@
 SecurityConfig *-- UserDetailsServiceImpl
 UserDetailsServiceImpl *-- SiteUserRepository
 SecurityController *-- SiteUserRepository
+UniqueLoginValidator *-- SiteUserRepository
+UniqueLogin .. UniqueLoginValidator
 SiteUserRepository <|-- SiteUser
 
 class City <<Entity>> {
diff --git a/src/main/java/osm/surveyor/task/user/model/SiteUser.java b/src/main/java/osm/surveyor/task/user/model/SiteUser.java
index 513cb2c..f7b0905 100644
--- a/src/main/java/osm/surveyor/task/user/model/SiteUser.java
+++ b/src/main/java/osm/surveyor/task/user/model/SiteUser.java
@@ -10,6 +10,7 @@
 
 import lombok.Getter;
 import lombok.Setter;
+import osm.surveyor.task.user.validator.UniqueLogin;
 
 @Getter
 @Setter
@@ -20,6 +21,7 @@
     private Long id;
 
     @Size(min = 2, max = 20)
+    @UniqueLogin
     private String username;
 
     @Size(min = 4, max = 255)
diff --git a/src/main/java/osm/surveyor/task/user/validator/UniqueLogin.java b/src/main/java/osm/surveyor/task/user/validator/UniqueLogin.java
new file mode 100644
index 0000000..3c05159
--- /dev/null
+++ b/src/main/java/osm/surveyor/task/user/validator/UniqueLogin.java
@@ -0,0 +1,18 @@
+package osm.surveyor.task.user.validator;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import javax.validation.Constraint;
+import javax.validation.Payload;
+
+@Target({ElementType.METHOD, ElementType.FIELD})
+@Retention(RetentionPolicy.RUNTIME)
+@Constraint(validatedBy = UniqueLoginValidator.class)
+public @interface UniqueLogin {
+    String message() default "このユーザ名は既に登録されています";
+    Class<?>[] groups() default{};
+    Class<? extends Payload>[] payload() default{};
+}
diff --git a/src/main/java/osm/surveyor/task/user/validator/UniqueLoginValidator.java b/src/main/java/osm/surveyor/task/user/validator/UniqueLoginValidator.java
new file mode 100644
index 0000000..4cf6aa5
--- /dev/null
+++ b/src/main/java/osm/surveyor/task/user/validator/UniqueLoginValidator.java
@@ -0,0 +1,27 @@
+package osm.surveyor.task.user.validator;
+
+import javax.validation.ConstraintValidator;
+import javax.validation.ConstraintValidatorContext;
+
+import org.springframework.beans.factory.annotation.Autowired;
+
+import osm.surveyor.task.user.repository.SiteUserRepository;
+
+public class UniqueLoginValidator implements ConstraintValidator<UniqueLogin, String> {
+
+    private final SiteUserRepository userRepository;
+
+    public UniqueLoginValidator() {
+        this.userRepository = null;
+    }
+
+    @Autowired
+    public UniqueLoginValidator(SiteUserRepository userRepository) {
+        this.userRepository = userRepository;
+    }
+
+    @Override
+    public boolean isValid(String value, ConstraintValidatorContext context) {
+        return userRepository == null || userRepository.findByUsername(value) == null;
+    }
+}