A method of indwelling a needle assembly includes providing an assembled indwelling needle assembly having an inner needle, an inner needle hub fixed to a proximal portion of the inner needle, a hollow outer needle in which the inner needle is inserted, an outer needle hub fixed to a proximal portion of the outer needle, and a pressing member; pressing down on the pressing member while moving the indwelling needle assembly to puncture a blood vessel; advancing the outer needle along the inner needle such that the outer needle is inserted in the blood vessel; holding the pressing member against the outer needle and holding the outer needle hub such that the outer needle is maintained indwelling in the blood vessel while simultaneously pulling the inner needle hub in a proximal direction such that the inner needle is pulled out from and entirely removed from the outer needle.