A gas generator includes, a housing having a defining a first combustion chamber, an ignition chamber cup, defining an ignition chamber, and a combustion chamber cup, defining a second combustion chamber, being aligned at a distance in a radial direction, a first communication hole communicating the ignition chamber with the first combustion chamber being provided in a circumferential wall portion of the ignition chamber cup, a second communication hole communicating the second combustion chamber with the first combustion chamber being provided in a circumferential wall portion of the combustion chamber cup, a cup-shaped cover member covering the combustion chamber cup and closing the second communication hole, a disk-shaped retainer retaining a gas generating agent, being provided with a first opening for fitting the ignition chamber cup and a second opening for fitting the combustion chamber cup, an annular gap being formed between the second opening and the cover member.