A head-protecting airbag of the present invention is folded and housed in upper edges of side windows, and is deployable downward when inflation gas flows in between its vehicle's inner wall and outer wall. The airbag includes a protective portion located at side of an occupant's head. The protective portion includes a temporary joint. The joint joins the inner wall and the outer wall together, and separates the inner wall and the outer wall when an inner pressure of the protective portion rises after the protective portion completes development. The airbag of the present invention assures a swift development in an initial stage of inflation, and after completing development, so inflates as to have sufficient cushioning property.