A cartridge adapter includes: a cartridge holder 103 having a space that can accommodate therein a drug cartridge, the drug cartridge including a cylinder 201 having a tubular internal space extending in a longitudinal direction, a gasket 202 supported in the internal space so as to be movable in the longitudinal direction, and a drug held in the internal space； a piston 109 that moves the gasket 202 in the longitudinal direction in the internal space of the cylinder 201； a piston guide 104 that movably supports the piston 109 and connected to the cartridge holder 103； and a piston driving mechanism 110 that drives the piston in the longitudinal direction.
