Abstract: An indwelling needle assembly includes an inner needle having a sharp needlepoint at a tip thereof, an inner needle hub fixed to an inner needle base, a hollow outer needle into which the inner needle is inserted, an outer needle hub fixed to the outer needle base, a tube connected to the base of the outer needle hub so that an inner cavity thereof communicates with an inner cavity of the outer needle, an opening formed at the base or the side of the outer needle so as to communicate with the inner cavity of the outer needle, a handling part, which is formed in the outer needle and by which the inner and outer needles are moved in a longitudinal direction thereof when the inner needle is inserted into the outer needle, and a locking means which is switchable, when the inner needle is inserted into the outer needle, between a locked state (i.e., the inner needle hub being locked to the outer needle hub) and an unlocked state (i.e., the locked state being released).