In a gas generator, a closing member includes: a first region exposed to a side of a second combustion chamber in a closed state, a second region in contact with a partition member from a side of the first combustion chamber in the closed state, and an engagement region that engages an engaged member fixed in a housing in the closed state; and the closing member is configured to have one or a plurality of communication holes be in a closed state by the closing member being supported at the partition member in the second region due to combustion pressure of a first gas generating agent and to have the closed state released by at least a part of the closing member, the closing member including a displacement portion that is formed continuously so as to be included at least a part of the first region, being displaced in a direction from the side of the second combustion chamber toward the side of the first combustion chamber by combustion pressure of a second gas generating agent acting on the first region.