A cartridge adapter includes: a cartridge holder having a space that can accommodate therein a drug cartridge, the drug cartridge including a cylinder having a tubular internal space extending in a longitudinal direction, a gasket supported in the internal space so as to be movable in the longitudinal direction, and a drug held in the internal space； a piston that moves the gasket in the longitudinal direction in the internal space of the cylinder； a piston guide that movably supports the piston and connected to the cartridge holder； and a piston driving mechanism that drives the piston in the longitudinal direction.
