A disposable syringe includes a plunger movable in a barrel relative to a needle seat that engages an inner surrounding barrel wall surface of the barrel by a resisting force. The plunger includes a plunger body and a coupling rod which is in frictional engagement with the plunger body and which has a central anchored area disposed to engage an anchoring portion of the needle seat by a holding force. As such, when the plunger body is moved to bring the central anchored area into engagement with the anchoring portion, the resisting force is overcome to permit disengagement of the needle seat from the barrel, and the coupling rod and the needle seat can be retracted into the plunger body by virtue of a biasing action of a biasing member.