A gas cartridge is firmly attached to a toy gun and the gas cartridge is replaced through simple operation. The gas cartridge is filled therein with compressed gas for firing a bullet. A cartridge housing portion slidably holds the circumferential surface of the gas cartridge. The lid portion of the gas cartridge housed in the cartridge housing portion is coupled to a cartridge attachment portion. A clamp arm has a roller and an extended portion and can be freely rotated around a rotation shaft. When the clamp arm is rotated, the roller presses the bottom portion of the gas cartridge in the cartridge housing portion. A raising lever has an abutment portion and is coupled to the extended portion so that the raising lever can be rotated around an installation shaft. When the raising lever is moved, the abutment portion slides on the sliding surface and rotates the clamp arm.