Abstract: Disclosed is an indwelling needle set comprising a cylinder provided with a flexible outer needle composed of a synthetic resin at the tip thereof and a plunger which is inserted in the cylinder and is provided with a metallic inner needle at the tip thereof, wherein the plunger is provided with a piston which slidingly-contacts the surface of the inside wall of the cylinder, an orifice which opens to the inside of the cylinder in front of the piston, a rear opening formed at the rear end of the cylinder for connecting a connecting tube, and a conduit penetrating through the plunger and connecting the orifice with the rear end opening; and the plunger is inserted into the cylinder in a manner that the piercing position at which the tip of the inner needle projects from the tip of the outer needle is allowed to slide relative to the housing position at which the inner needle is housed after it is backed away.