The present invention provides a gas generator including: an igniter, a collar for holding the igniter, and a cup-shaped case fixed to the collar and charged inside with gas generating agent, the collar formed by a combination of a metallic portion and a resin portion which is integrated with the metallic portion, the collar being provided with an annular groove formed in an outer peripheral surface thereof, the cup-shaped case and the collar being fixed by fitting a bent portion, which is obtained by bending inward an end portion in an opening side of the cup-shaped case, to the annular groove, an annular space being formed by an inner wall surface of the bent portion and the collar, and being in communication with an inside of the cup-shaped case.