Abstract: Disclosed is an improved medical device comprising: a needle assembly comprising a needle body having a needle hub fixed to its needle; a hollow casing at least opening at its top for slidably containing the needle assembly, the opening top of the hollow casing having inward projections on its inner circumference; a stopper movably attached to the vicinity of the opening top of the hollow casing; and a protection cover for enclosing at least the sharp point of the needle, thereby permitting the needle assembly to be retained in the hollow casing by putting the stopper in the shutting position, or permitting the needle assembly to appear from the hollow casing by holding the needle hub of the inner needle between the inward projections of the opening top and the stopper, which is put in the shutting position.