To provide a seat that enhances the stability and expansion speed with respect to the inflation expansion performance of airbag unit disposed thereinside. In one form, a seat is provided comprising a backrest part covered at its entirety with top fabric material and, at a given region of the top fabric material, it is furnished with tear line; an airbag constituted of a bag consisting of two basis fabrics joined together and disposed within the backrest part so as to at the time of inflation, cleave the tear line and expand over the head of driver or passenger; and reinforcing fabric disposed on the top fabric material so as to inhibit any cleavage of the top fabric material at regions other than the tear line.