Drug injection device, cartridge adapter, and drug injection system

A drug injection system comprising a cartridge adapter and a drug injection device to which the cartridge adapter is attached, wherein:

• the cartridge adapter includes:
• an elongate tubular main body case including a needle attachment portion on a front end side thereof;
• an attachment portion provided on the main body case used for attachment to the drug injection device;
• an accommodating portion that accommodates therein a drug cartridge having a gasket on an inner side thereof;
• a piston that opposes the gasket, with the drug cartridge accommodated in the accommodating portion;
• a piston gear having a piston drive thread on an inner circumference thereof that allows the piston to slide therethrough, and connection teeth on an outer circumference thereof that are connected to the drug injection device;
• a stopper that stops movement of the piston gear;
• and
• a release lever that releases a locked state of the piston gear;
• and
• the drug injection device includes:
• an elongate tubular casing having an accommodating space that accommodates therein a portion of the cartridge adapter;
• an engagement portion provided in the casing that engages with the cartridge adapter;
• a drive gear to which the piston gear of the cartridge adapter is connected;
• a motor connected to the drive gear;
• and
• a release bar that drives the release lever of the cartridge adapter.