A kind of injector for medical purpose, injector housing, plunger rod and the plug engaged with a part of plunger rod including limiting reservoir, plug are slidably disposed on the sealing engagement for being dimensioned to offer with the side wall of injector housing of the inside in reservoir and relative to injector housing. Injector for medical purpose further includes the first cap of the sharp first end of the needle with sharp first end and the second end being connected to reservoir, covering needle and at least part of second cap for covering the first cap. Second cap includes the ontology with proximal end, distal end and the hemispherical member including holding part. Second cap is engaged with the first cap, so that the second cap removes while removes the first cap from the first end of needle.
