Method for manufacturing air bag, air bag, and seat device for vehicle

Provided is a method for manufacturing an air bag having excellent durability. For an air bag (40), a welding die (60) is pressed against two sheets (41, 42) arranged in an overlapping manner from the overlapping direction of the sheets (41, 42), thereby forming a bag part (45) surrounded by a welding part (50) extending in a ring shape. Furthermore, the deposition die (60) is pressed against the outer surface (42s) of the sheet (42), thereby forming a deposition section (50) of the two sheets (41, 42), the deposition section (50) having a deposition bead section (80) between the two sheets (41, 42), the deposition bead section (80) protruding toward the inside of the pocket section (45). Furthermore, on the pressing surface (70) of the deposition die (60) with respect to the sheet (42), an inclined surface section (75) facing the overlapping direction of the sheets (41, 42) and the direction of the inside of the pocket section (45) is provided on the outer side of the pocket section (45) with respect to the