An airbag in an uninflated-and-spread form includes a rear-lower inflation portion and a front-upper inflation portion. The airbag is stored in a first vehicle seat. The vehicle seat adjacent to the first vehicle seat is a second vehicle seat. The front-upper inflation portion includes at least three bend portions, which are arranged in a flowing direction of inflation gas. A form between the uninflated-and-spread form and the storage form is a transitional form. In the transitional form, the bend portions are each bent relative to the adjacent bend portion and rolled toward the second vehicle seat relative to the rear-lower inflation portion such that the closer a bend portion is to the downstream end, the more inward the bend portion is located. In the transitional form, the upper end of the most upstream bend portion is connected to the upper end of the rear-lower inflation portion.