A disposable syringe includes a barrel (1), a needle assembly (2), a plunger (3), a press-fitting unit (51,52), and a spacer (4). The plunger (3) is movable in the barrel (1) to a retraction initiating position, where a front open plunger end (37) is coupled with the needle assembly (2) for withdrawal of the needle assembly (2) into the plunger (3). An endcap (33) is disposed to cover a rear open plunger end (38) of the plunger (3). The press-fitting unit (51,52) is disposed between the plunger (3) and an inner barrel surface (130) of the barrel (1) to permit press-fit engagement between the plunger (3) and the barrel (1) immediately after the plunger (3) is moved to the retraction initiating position. The spacer (4) includes front and rear abutment surfaces (411,412) detachably engaged with the barrel (1) and the endcap (33) in a preuse position.