An air bag has an air bag main body and a sheet. The air bag main body is formed into such a sac like shape as having a circumferential wall portion that extends in a sleeve-like form from a peripheral edge of an opening and a bottom wall portion that closes the circumferential wall portion. The sheet is arranged so as to extend from the peripheral edge of the opening to the bottom wall portion via the circumferential wall portion in such a manner that the air bag main body is split into two parts, and the sheet has a communicating hole. The sheet in sewn only to a portion of the peripheral edge of the opening in the circumferential wall portion and to a portion of the bottom wall portion.