A gas cartridge loading mechanism has a sensor member movable toward a collar retaining portion and mounted to undergo pivotal movement between a locked position and an unlocked position, and a stopper configured to prevent movement of the sensor member when the sensor member is disposed in the locked position and to allow movement of the sensor member when the sensor member is disposed in the unlocked position. The sensor member is configured to move in the locked position when a gas cartridge is set with improper orientation, and to move in the unlocked position when the gas cartridge is set with proper orientation.