The thickness (4W) of a side seal portion (4) is greater as compared to the thickness of a ventral portion (F) and a dorsal portion (R) which constitute this portion, in respective sections thereof other than the side seal portion (4). The ventral portion (F) and the dorsal portion (R) which constitute the side seal portion (4) each include a laminated structure of a plurality of sheet members, wherein the constituent fibers of at least one of sheet member from among the plurality of sheet members has a melting point different from the melting point of the constituent fibers of the other sheet members. The laminated structures of the ventral portion (F) and the dorsal portion (R) each include a low-melting point innermost layer type laminated structure (41) comprising a low-melting point sheet member (37) in which the innermost layer has the lowest melting point among the plurality of sheet members. The side seal portion (4) is formed by a laser sealing process.
