To provide a drug solution preparing kit which can be handled with ease in a substantially aseptic manner, and has no risk that a leakage of a drug solution such as a splash and a dispersion of an aerosol to an ambient environment occurs upon preparation of the drug solution There is provided a drug solution preparing kit including a pre-filled syringe and a transfusing tool, wherein the pre-filled syringe includes a sealing member which seals the tip end and can not be removed from the tip end, and that the transfusing tool includes a one-way valve which can discharge only gas from the system in an irreversible manner, and a filter which is provided so as to adjoin to the second communication channel with respect to the one-way valve.