A drug solution filling unit comprising: a drug solution reservoir instrument having a drug solution reservoir part that is configured to reserve a drug solution; and a filling adapter configured to be detachably mounted on the drug solution reservoir instrument, wherein the drug solution reservoir instrument includes: the drug solution reservoir part having a filling port from which the drug solution can be injected; and an exterior case that is configured to house the drug solution reservoir part, and the filling adapter includes: a liquid supply needle tube configured to be connected to the 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 can pass; characterized by the drug solution reservoir part further having a liquid supply port that is configured to send the filled drug solution; and the filling adapter further including: an air vent needle tube configured to be connected to a drug solution container and through which gas that is sent to the drug solution container can pass; and a suction channel configured to communicate with the liquid supply port and configured to communicate with a suction mechanism that is configured to suck in gas within the drug solution reservoir part. a drug solution reservoir instrument having a drug solution reservoir part that is configured to reserve a drug solution; and a filling adapter configured to be detachably mounted on the drug solution reservoir instrument, wherein the drug solution reservoir instrument includes: the drug solution reservoir part having a filling port from which the drug solution can be injected; and an exterior case that is configured to house the drug solution reservoir part, and the filling adapter includes: a liquid supply needle tube configured to be connected to the 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 can pass; characterized by the drug solution reservoir part further having a liquid supply port that is configured to send the filled drug solution; and the filling adapter further including: an air vent needle tube configured to be connected to a drug solution container and through which gas that is sent to the drug solution container can pass; and a suction channel configured to communicate with the liquid supply port and configured to communicate with a suction mechanism that is configured to suck in gas within the drug solution reservoir part. the drug solution reservoir part having a filling port from which the drug solution can be injected; and an exterior case that is configured to house the drug solution reservoir part, and the filling adapter includes: a liquid supply needle tube configured to be connected to the 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 can pass; characterized by the drug solution reservoir part further having a liquid supply port that is configured to send the filled drug solution; and the filling adapter further including: an air vent needle tube configured to be connected to a drug solution container and through which gas that is sent to the drug solution container can pass; and a suction channel configured to communicate with the liquid supply port and configured to communicate with a suction mechanism that is configured to suck in gas within the drug solution reservoir part. a liquid supply needle tube configured to be connected to the 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 can pass; characterized by the drug solution reservoir part further having a liquid supply port that is configured to send the filled drug solution; and the filling adapter further including: an air vent needle tube configured to be connected to a drug solution container and through which gas that is sent to the drug solution container can pass; and a suction channel configured to communicate with the liquid supply port and configured to communicate with a suction mechanism that is configured to suck in gas within the drug solution reservoir part. an air vent needle tube configured to be connected to a drug solution container and through which gas that is sent to the drug solution container can pass; and a suction channel configured to communicate with the liquid supply port and configured to communicate with a suction mechanism that is configured to suck in gas within the drug solution reservoir part.