A restricting member 39 disposed along an outer surface of an air bag 32 restricts an axial inflation of the air bag 32 in an earlier stage of deployment of the air bag 32, and in a later stage of deployment, the restricting member 39 allows for the inflation of the air bag 32 to its maximum capacity through breakage of brittle portions 41b. In the deployment process of the air bag 32, since vent holes 37b formed in a base fabric 37 of the air bag 32 are closed by the restricting member 39, the leakage of gas from the vent holes 37b is prevented to thereby enable a quick deployment of the air bag 32. Then, when the brittle portions 41b on the restricting member 39 break, the vent holes 37b are opened.