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