In a side airbag system (10), a tether portion (22), which serves as a partition between an upper bag portion (16) and a lower bag portion (18), is formed of upper regions (34U) of a piece of lower base fabric (34), and is formed by sewing the upper regions (34U) of the lower base fabric (34), folded toward a vehicle front side along a fold axis, together. In addition, a tube (24) that distributes inflation gas between the upper bag portion (16) and the lower bag portion (18) extends through the tether portion (22) to the lower bag portion (18) and to the upper bag portion (16). The tube (24) is formed into a cylindrical shape in such a manner that a piece of tube base fabric (36) is sewed to the upper regions (34U) of the lower base fabric (34) and folded toward a vehicle front side along the fold axis and then front ends (36F) of the folded tube base fabric are sewed together in a tube longitudinal direction.