A drug solution filling unit comprising: a drug solution reservoir instrument having a drug solution reservoir portion which stores a drug solution; a filling adapter detachably mounted to the drug solution reservoir instrument; and a lock mechanism which restricts a relative movement of the filling adapter with respect to the drug solution reservoir instrument; wherein the drug solution reservoir instrument has an exterior case (210, 410) which accommodates the drug solution reservoir portion, a first port portion provided in the drug solution reservoir portion, and a first seal member disposed in the first port portion; the filling adapter has a first needle tube connected to a drug solution container containing the drug solution and that is configured to pierce to the first seal member, and an adapter main body portion to which the first needle tube is attached, characterized in that the lock mechanism is able to hold a relative position of the adapter main body portion with respect to the exterior case (210, 410), at a non-penetrating position at which the first needle tube does not penetrate the first seal member, and a penetrating position at which the first needle tube penetrates the first seal member, in a state in which the filling adapter has been mounted to the drug solution reservoir instrument; the lock mechanism is provided integrally with the filling adapter, releases the restriction with an operation of moving the filling adapter to relatively slide with respect to the drug solution reservoir instrument, and enables movement from the non-penetrating position to the penetrating position; the lock mechanism has a claw portion which restricts the relative movement of the filling adapter with respect to the drug solution reservoir instrument by being caught by a groove portion (217b, 217c) formed in the exterior case (210, 410): and the lock mechanism has a lever portion (361, 453) which releases the catching of the claw portion with respect to the groove portion (217b, 217c) with a pressing operation. a drug solution reservoir instrument having a drug solution reservoir portion which stores a drug solution; a filling adapter detachably mounted to the drug solution reservoir instrument; and a lock mechanism which restricts a relative movement of the filling adapter with respect to the drug solution reservoir instrument; wherein the drug solution reservoir instrument has an exterior case (210, 410) which accommodates the drug solution reservoir portion, a first port portion provided in the drug solution reservoir portion, and a first seal member disposed in the first port portion; an exterior case (210, 410) which accommodates the drug solution reservoir portion, a first port portion provided in the drug solution reservoir portion, and a first seal member disposed in the first port portion; the filling adapter has a first needle tube connected to a drug solution container containing the drug solution and that is configured to pierce to the first seal member, and an adapter main body portion to which the first needle tube is attached, characterized in that a first needle tube connected to a drug solution container containing the drug solution and that is configured to pierce to the first seal member, and an adapter main body portion to which the first needle tube is attached, the lock mechanism is able to hold a relative position of the adapter main body portion with respect to the exterior case (210, 410), at a non-penetrating position at which the first needle tube does not penetrate the first seal member, and a penetrating position at which the first needle tube penetrates the first seal member, in a state in which the filling adapter has been mounted to the drug solution reservoir instrument; the lock mechanism is provided integrally with the filling adapter, releases the restriction with an operation of moving the filling adapter to relatively slide with respect to the drug solution reservoir instrument, and enables movement from the non-penetrating position to the penetrating position; the lock mechanism has a claw portion which restricts the relative movement of the filling adapter with respect to the drug solution reservoir instrument by being caught by a groove portion (217b, 217c) formed in the exterior case (210, 410): and the lock mechanism has a lever portion (361, 453) which releases the catching of the claw portion with respect to the groove portion (217b, 217c) with a pressing operation.