A gas generator includes a housing and a diffuser connected to the housing. The diffuser has a cup-like shape and includes an opening portion joined to the housing and a circumferential wall portion provided with gas discharge ports. A partition plate is attached to the opening between the housing and the diffuser for closing the opening. The diffuser has first and second tubular space portions therein communicating with each other, the first tubular space portion facing the partition plate, and the second tubular space portion communicating with the gas discharge ports. A step is formed between the first and second tubular space portions such that the inner diameter of the first tubular space portion is larger than the inner diameter of the second tubular space portion, and a cross-sectional area of the second tubular space portion is smaller than the total opening area of the gas discharge ports.