A bullet cartridge for toy air gun (air soft gun BB-gun) in which a gasket member is mounted with a metal bush and stopped at an annular stop flange between a front air chamber and a rear air chamber to support a reciprocating motion of a piston rod, and a compression spring is mounted on the piston rod and stopped between the metal bush and a collar of the piston rod to hold the piston rod in the close position where the front stop flange of the piston rod is stopped at the front side of the gasket member to seal the air passage between the CO.sub.2 steel cylinder and the jet nozzle through which compressed gas passes to drive a bullet out of the toy air gun.