A gas generator includes an igniter, a cup-shaped member, and a fixing member. The cup-shaped member accommodating an enhancer agent bursts or melts at the time of activation. The fixing member fixing a filter does not burst and melt even at the time of activation. Ra and Ha satisfy a condition of Ra/Ha 1.00 where Ra represents an inner diameter of a sidewall portion of the cup-shaped member and Ha represents a distance between a top wall portion of the cup-shaped member and the igniter. The sidewall portion includes a first region surrounded by a partition wall portion of the fixing member and a second region not surrounded by the partition wall portion. The partition wall portion has an end portion arranged closer to a top plate portion than the igniter. A gas generating agent is arranged to face the top wall portion, the second region, and the partition wall portion.