A drug solution filling unit includes a drug solution reservoir instrument and a filling adapter. The drug solution reservoir instrument includes a drug solution reservoir part that reserves a drug solution. The drug solution reservoir part has a filling port and a liquid supply port. The filling adapter includes an air vent needle tube, a liquid supply needle tube, and a suction channel. The liquid supply needle tube is connected to a drug solution container and the filling port and through which the drug solution that is sent from the drug solution container to the drug solution reservoir part passes. The suction channel communicates with the liquid supply port and communicates with a suction mechanism that sucks in gas within the drug solution reservoir part.
