A retainer case (4) has a lower portion (4a) defining a gas diffusion chamber (5), and an upper portion (4c) cooperating with an intermediate portion (4b) to define an accommodation chamber (6), a diffuser member (15) separates the gas diffusion chamber (5) from the air bag accommodation chamber (6), with gas communication allowed therebetween through a gas inlet (16) of the diffuser member (15), an infaltor (11) is disposed in the gas diffusion chamber (5), an airbag (8) is accommodated in the accommodation chamber (6), having a retained part (8b) in a space (S1) between the intermediate portion (4b) and the diffuser member (15), and an inward projection (18) of the intermediate portion (4b) and an inward projection (17) of the diffuser member (15) constitute a gas invasion restricting structure (17+18) for restricting a gas invasion from the gas diffusion chamber (5) into the space (S1).