To reduce the cost using a small-sized inflator and an inflator bag and accomplish sufficient restraint of waist even if the small-sized inflator and the inflator bag are used.

A waist restraining device A to protect a seat occupant in the event of vehicle collision by restraining the occupant at the seat is equipped with the inflator 7 to jet high pressure gas and the inflator bag 6 to be spread by the high pressure gas, in which a protrusive space body 11 in triangular shape to be formed when the inflator bag is spread, is composed of a bottom surface plate 1 whereon the inflator bag 6 is mounted, a front face plate 2 installed rotatably on the front end 1a of the bottom surface plate 1 to be raised when the inflator bag is spread, and a cover plate 3 installed rotatably on the tail 1b of the bottom surface plate 1 to be raised when the inflator bag is spread.