The invention includes a barrel, a needle assembly, a catheter, a stopper and a vacuum generator. The needle assembly is disposed in the barrel and has a needle, a needle hub and a needle piston. The needle piston is fastened onto the needle hub. The catheter is axially connected onto the needle hub and passed through by the needle. The stopper is operably disposed at the first end of the barrel and normally stops the needle hub from moving. The vacuum generator has a plunger and a vacuum piston mounted on an end thereof. The vacuum piston is slidably received in the barrel and normally abuts against the needle piston, and the plunger protrudes from the second end of the barrel. The needle assembly will be pulled and retracted in the barrel when the plunger is pulled outward and the stopper releases the needle hub.
