The airbag includes a bag-shaped circumferential wall formed of base cloths, and a lap portion. In the lap portion, a terminal portion of a downstream side base cloth, which is located in downstream of inflation gas, is superposed on a terminal portion of an upstream side base cloth, which is located in upstream of inflation gas. Then the lapping terminal portions are joined to each other at both sides thereof in a direction extending along edges of the terminal portions, thereby providing the lap portion. A communication port communicating inside and outside of the airbag is provided between the joined portions and between the terminal portions of the upstream and the downstream side base cloths. With this construction, in the course of airbag inflation, inflation gas flowing toward the downstream side base cloth from the upstream side base cloth sucks air into the airbag from outside via the communication port, and when the airbag bumps against a target object of protection halfway of inflation, inflation gas is exhausted out of the airbag via the communication port. Moreover, when the airbag is completely inflated, the communication port is closed with the terminal portions contacted with each other. The airbag of the present invention suppresses increase of internal pressure upon engaging a target object of protection halfway of inflation, as well as completing inflation swiftly.