disposed adjacent to the inner side of the engagement blocks 27 and adapted to stop against the stop rods 18 of the barrel 10 the needle holder 30 has a holder body 31 adapted to hold a needle cannula 80 and a locating groove 34 extended around the periphery of the holder body 31 and adapted to receive the pawls 14 of the barrel 10 the plunger 40 has an elongated plunger body 41 a stopper retainer 42 disposed at the front end of the plunger body 41 and a thumb rest 49 provided at the rear end of the plunger body 41 for the rest of the thumb the rubber stopper 50 has a stopper body 51 fastened to the stopper retainer 42 and supported on the front end of the plunger body 41 and peripherally disposed in close contact with the inside wall the peripheral wall of the receiving hole 12 of the inner barrel 10 referring to fig2 and 3 by means of the through hole 22 the rotary chuck 20 is sleeved onto the end stop 16 of the barrel 10 for enabling the ribs 25 of the rotary chuck 20 to be stopped at the extension portions 15 of the barrel 10 when sleeving the rotary chuck 20 onto the end stop 16 of the barrel 10 the stop blocks 28 are forced to deform and to pass over the end stop 16 when the stop blocks 28 moved over the end stop 16 the rotary chuck 20 is prohibited from backward movement relative to the barrel 10 i e the end stop 16 stops the rotary chuck 20 from.