外部キー制約

public function up()
{
    Schema::create('users', function (Blueprint $table) {
        $table->increments('id')->unique('pk_s_id');
        $table->string('name', 255)->nullable(false);
        $table->string('kana', 255)->->nullable(false);
        $table->timestamp('updated_at')->useCurrent()->nullable(false);
        $table->timestamp('created_at')->useCurrent()->nullable(false);
         $table->foreign('User_id')->references('id')->on('company');
    });
}