A passenger seat airbag includes a passenger side wall for catching a passenger, and a circumferential wall that extends forward from the passenger side wall in a converging fashion. The circumferential wall includes an upper wall, a lower wall, a left wall and a right wall, and can be mounted on a housing by the front end. An inlet port for introducing inflation gas is disposed at a front end region of the lower side wall, at a position displaced to the right from the center in a left and right direction of the passenger side wall. In the front end region of the circumferential wall, with respect to a substantial circumferential length which permits free expansion, a left circumferential wall region disposed on the left of the inlet port and a right circumferential wall region disposed on the right of the inlet port have equal substantial circumferential lengths.