A airbag main body is formed by sewing a pair of fabric sheets. The airbag main body is provided with a seam. The seam has passages for permitting gas in the airbag main body to pass therethrough. The seam divides the interior of the airbag main body into a main inflation chamber and a sub-inflation chamber. The sub-inflation chamber is inflated with gas that has flowed from the main inflation chamber and passed through the passages formed in the seam.