To provide a drug solution filling unit, a drug solution filling set, and a filling adapter capable of preventing a decrease in the sealing property of a seal member of a drug solution reservoir portion and easily performing an operation of causing a needle tube to penetrate the seal member of the drug solution reservoir portion.A drug solution filling unit 100 includes a drug solution reservoir instrument 200 having a drug solution reservoir portion 230 for storing a drug solution, a filling adapter 300 detachably mounted to the drug solution reservoir instrument, and a lock mechanism 360 for restricting the relative movement of the filling adapter with respect to the drug solution reservoir instrument. The lock mechanism is able to hold the relative position of an adapter main body portion 310 with respect to an exterior case 210 at a non-penetrating position at which a first needle tube does not penetrate a seal member, and a penetrating position at which the first needle tube penetrates the seal member, in a state in which the filling adapter has been mounted to the drug solution reservoir instrument.
