An igniter 26 accommodated in a diffuser portion 20 has, on a peripheral face of a cup 28 covering an igniting portion, a hole 29 covered with a sealing tape from the inside of the cup, and the hole 29 exactly opposes the rupturable plate 19. When the igniter 26 is actuated, a rupturing energy is intensively discharged from the hole 29 to the rupturable plate 19, so that the rupturing ability of the rupturable plate 19 can be improved.