A syringe includes a barrel having a mouth on one end portion, a needle seat and a needle engaged in the mouth of the barrel, a plunger slidably engaged into the barrel, a packing releasably engaged in the plunger for forming a vacuum within the plunger, and an engaging member attached to the packing and extended out of the plunger for engaging with and for moving the needle seat and the needle into the plunger, and the packing and the engaging member and the needle are forced to move into the plunger by the vacuum of the plunger when the packing is forced to move relative to the plunger and when a releasable engagement between the plunger and the packing is spoiled.
