To provide a flap stopper of a bag, the stopper having a simple structure, small number of components, light weight and a superior operability.

The flap stopper 10 of a bag is composed of a male part 11 and a female part 12 that are provided in a bag body 2 and an extended part 5 of a hanging flap 3, wherein the hanging flap 3 is restrained to the bag body 2 by the male/female connection of the male part 11 and the female part 12. The male part 11 has an N-pole magnet 24, with a projected part 21 provided having a groove 22 in the peripheral face. The female part 12 comprises: a housing 26 provided with a recessed part 27 having a diameter slightly larger than the projected part 21, with a bottom face formed in the recessed part 27; an S-pole magnet 34 in the bottom face; and a slide member 29 which is slidably moved by a pressurizing force A and which is provided with a peg 32a that is energized in a direction opposite from the direction added by the pressurizing force A. The male part 11 and the female part 12 are configured to be attracted with a magnetic force at the time of female/male connection. At the time of release of the female/male connection, the slide member 29 shifts to cut off the attracting magnetic force.