The present invention provides a gas generator including a metallic first collar of a first igniter of an ignition device chamber, which has a first columnar main body portion, a first plate portion extending in a radial direction of a housing from part of a circumferential surface thereof, and a first wall surface formed by extending part of the first plate portion in an axial direction of the housing, a metallic second collar of a second ignition device of a second combustion chamber, which has a second columnar main body portion, a second plate portion extending in the radial direction from part of a circumferential surface thereof and a second wall surface formed by extending part of the second plate portion in the axial direction of the housing, a bottom side of the first columnar main body portion portion of the first collar being fitted in a first hole to fix a first igniter of the ignition device chamber, the first hole being one of two holes formed in the bottom plate of the housing separately from each other in the radial direction, a bottom side of the second columnar main body portion portion of the second collar being fitted in a second hole to fix a second igniter of the second combustion chamber, the second hole being the other one of the two holes formed in the bottom plate separately from each other in the radial direction, further, the first wall surface of the first collar and the second wall surface of the second collar being arranged to face each other in the radial direction, when a pressure inside the housing rises at a time of actuation, the first wall surface and the second wall surface being in contact with each other in a state of being pushed radially inward against each other.