A cartridge-based air gun that fires a BB bullet or a resin bullet inside a cartridge using compressed gas, includes an engaging structure of an extractor that enables blowback and the automatic ejection of the cartridge. An extractor, which engages with a rim that is formed at the rear end of a cartridge loaded in a chamber, is pivotally provided on a side portion of a cylinder so as to be rotatable. A claw portion formed at the tip of the extractor engages with the rim of the cartridge by the urging force of a spring provided between the extractor and the cylinder. A slide moves rearward while the cartridge is held inside the chamber. As the cylinder moves rearward with the stopping of the supply of compressed gas from a gas reservoir, the cartridge comes into contact with an ejector, is released from the engagement by the extractor, and is ejected from the chamber.