An airbag is installed in a seat, and since an airbag cushion is deployed from an upper side of the seat toward the front of a passenger to expand so as to cover the passenger, the upper body of the passenger is stably restrained. In addition, the airbag cushion is deployed while advancing forward so as not to be interfered with by the passenger by the guide wire, and the binding force of the airbag cushion is secured by the guide wire so that the protection performance for the passenger via the airbag cushion is improved.