A method includes: providing an assembly including: an inner needle, an inner needle hub, a catheter, a catheter hub, a catheter operation member, and a support member; puncturing a blood vessel of a patient with the inner needle and the catheter, while the support member is at a first position in which the support member supports the catheter; advancing the catheter operation member relative to the needle hub such that the catheter advances in the blood vessel, such that a part of the support member is caused to deform relative to the needle hub as the support member moves from the first position to a second position in which the support member does not support the catheter, and such that the catheter hub detaches from the needle hub; and retracting the inner needle hub and the inner needle relative to the catheter hub and the catheter.