Dual-stage-output inflator for airbag, and vehicle airbag device

In order to suppress the circulation of gas from one combustion chamber to the other in a first combustion chamber and a second combustion chamber, an inflator (10) is provided with: a cup-shaped lower case (12); an inverted cup-shaped first upper case (14), an end part (14C) on the open side thereof being fixed to an end part (12E) on the open side of the lower case (12), and the first upper case and the lower case (12)forming a first combustion chamber (18) for combusting a first gas forming agent (20); and an inverted cup-shaped second upper case (16) provided on the opposite side of the first upper case (14) from the lower case (12), an end part (16C) on the open side of the second upper case being fixed to the first upper case (14), and the second upper case and a first ceiling wall (14B) of the first upper case (14) forming a second combustion chamber (22) for combusting a second gas forming agent (24).