A disposable syringe includes a tubular needle seat inserted into a barrel for fixing a needle cannula. The needle seat has an axially extending cavity with a radially yieldable catch disposed therein. A plunger is movable in the barrel, and has an engaging head defining a rearwardly facing shoulder wall which is retained by the catch when the engaging head is extended into the cavity by a pushing force applied to the plunger. The barrel defines friction diminishing regions such that when the needle seat is moved past the friction diminishing regions to be placed to a disposal position, friction between the needle seat and the barrel is diminished, thereby facilitating a subsequent pulling action of the plunger for retracting the needle cannula into the barrel.