The present invention provides a gas generator, including, a housing having a gas discharge port, a combustion chamber cup member disposed, in the housing, such that an opening portion thereof covers a bottom surface of the housing, the combustion chamber cup member having a communication hole at least a portion of its peripheral wall surface thereof, a metallic cover member covering the peripheral wall surface provided with the communication hole, and a combustion chamber being formed inside the combustion chamber cup member and accommodating a gas generating agent and an igniter therein, at the time of activation, the cover member expanding and deforming mainly in the radial direction to form a gap between the peripheral wall surface of the combustion cup member and the metallic cover member, which serves as a gas channel leading to the combustion chamber and the outside of the combustion chamber.