A gas generator includes a housing containing an accommodation space, a gas generating agent, an igniter, and a sealing tape. The housing includes a cylindrical circumferential wall portion including plural gas discharge openings and the plural gas discharge openings are closed by the sealing tape. The plural gas discharge openings include gas discharge openings set to be different from one another in opening pressure so as to be opened stepwise with increase in pressure in the accommodation space as a result of burning of the gas generating agent. A condition of T≤1.8 [mm] and SA≥60 [mm2] is satisfied where T represents a thickness of a portion of the housing and SA represents a total opening area of the plurality of gas discharge openings. At least one of the plural gas discharge openings is in a shape of an elongated hole.