diff --git a/app/Http/Controllers/Web/Contract/CreateParam.php b/app/Http/Controllers/Web/Contract/CreateParam.php
index 1682245..f6db089 100644
--- a/app/Http/Controllers/Web/Contract/CreateParam.php
+++ b/app/Http/Controllers/Web/Contract/CreateParam.php
@@ -8,6 +8,7 @@ use App\Models\Contract;
 /**
  * @property string $name
  * @property ?string $custom
+ * @property ?string $contractCode
  */
 
 class CreateParam extends BaseParam
@@ -18,6 +19,7 @@ class CreateParam extends BaseParam
             [
                 Contract::COL_NAME_NAME => $this->str(),
                 Contract::COL_NAME_CUSTOM => $this->str(true),
+                Contract::COL_NAME_CONTRACT_CODE => $this->str(true),
             ];
     }
 }
diff --git a/app/Logic/Contract/ContractManager.php b/app/Logic/Contract/ContractManager.php
index 2e304f2..4ed0307 100644
--- a/app/Logic/Contract/ContractManager.php
+++ b/app/Logic/Contract/ContractManager.php
@@ -57,4 +57,31 @@ abstract class ContractManager
         $this->contract->save();
         return $this;
     }
+
+    /**
+     * 契約識別子がUNIQUE判定する
+     *
+     * @param array $messages
+     * @param boolean $forUpdate
+     * @return boolean
+     */
+    protected function checkContractCode(array  &$messages, bool $forUpdate): bool
+    {
+        $contractCode = $this->contract->contract_code;
+
+        if ($contractCode === null) {
+            return true;
+        }
+
+        $query = Contract::whereContractCode($contractCode);
+        if ($forUpdate) {
+            $query->whereNot(Contract::COL_NAME_ID, $this->contract->id);
+        }
+
+        if ($query->exists()) {
+            $messages[Contract::COL_NAME_CONTRACT_CODE] = "すでに使われています";
+            return false;
+        }
+        return true;
+    }
 }
diff --git a/app/Logic/Contract/CreateManager.php b/app/Logic/Contract/CreateManager.php
index 06673ca..209fe04 100644
--- a/app/Logic/Contract/CreateManager.php
+++ b/app/Logic/Contract/CreateManager.php
@@ -7,8 +7,24 @@ class CreateManager extends ContractManager
     public function create(): array
     {
         $messages = [];
+
+        if (!$this->checkForCreate($messages)) {
+            return $messages;
+        }
+
         $this->save();
 
         return $messages;
     }
+
+    private function checkForCreate(array &$messages): bool
+    {
+
+        $messages = [];
+        $ret = true;
+
+        $ret = $this->checkContractCode($messages, false);
+
+        return $ret;
+    }
 }
diff --git a/app/Models/Contract.php b/app/Models/Contract.php
index 6ee9597..05217f5 100644
--- a/app/Models/Contract.php
+++ b/app/Models/Contract.php
@@ -10,6 +10,7 @@ class Contract extends AppModel
 
     const COL_NAME_NAME = 'name';
     const COL_NAME_CUSTOM = 'custom';
+    const COL_NAME_CONTRACT_CODE = 'contract_code';
 
     public function getModelName(): string
     {
diff --git a/app/Repositories/ContractRepository.php b/app/Repositories/ContractRepository.php
index 1d1588c..6cc380e 100644
--- a/app/Repositories/ContractRepository.php
+++ b/app/Repositories/ContractRepository.php
@@ -65,6 +65,7 @@ class ContractRepository extends BaseRepository
             $this->makeColumnNameForSelect([$contract, Contract::COL_NAME_ID]),
             $this->makeColumnNameForSelect([$contract, Contract::COL_NAME_NAME]),
             $this->makeColumnNameForSelect([$contract, Contract::COL_NAME_CUSTOM]),
+            $this->makeColumnNameForSelect([$contract, Contract::COL_NAME_CONTRACT_CODE]),
             $this->makeColumnNameForSelect([$contract, Contract::COL_NAME_UPDATED_AT]),
             $this->makeColumnNameForSelect([$contract, Contract::COL_NAME_CREATED_AT]),
         ];
diff --git a/database/migrations/2023_08_22_100200_add_column_contracts_contract_code.php b/database/migrations/2023_08_22_100200_add_column_contracts_contract_code.php
new file mode 100644
index 0000000..6d75cbb
--- /dev/null
+++ b/database/migrations/2023_08_22_100200_add_column_contracts_contract_code.php
@@ -0,0 +1,44 @@
+<?php
+
+use App\Util\MigrationHelper;
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up()
+    {
+        Schema::table('contracts', function (Blueprint $table) {
+            $helper = new MigrationHelper($table);
+            $table->string("contract_code")->nullable()->comment("契約識別子");
+            $helper->unique(1, ['contract_code']);
+        });
+        Schema::table('contract_histories', function (Blueprint $table) {
+            $table->string("contract_code")->nullable()->comment("契約識別子");
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     *
+     * @return void
+     */
+    public function down()
+    {
+        Schema::table('contracts', function (Blueprint $table) {
+            $helper = new MigrationHelper($table);
+            $table->dropColumn("contract_code");
+
+            $helper->dropIndex(1);
+        });
+        Schema::table('contract_histories', function (Blueprint $table) {
+            $table->dropColumn("contract_code");
+        });
+    }
+};