Abstract: An improved structure safety syringe comprised of a barrel having two trapezoidal-shaped tabs, a sleeve with a guide slot and, furthermore, limit slots extending from the circumferential surface at the two ends of the guide slot and an insertion section is formed at the inside end of the limit slot, wherein utilizing the engagement of the tabs and the insertion section enables the positioning of the barrel and the sleeve, and a plunger having a protective cap at the force application end that can be broken off at an appropriate time. When the protective cap is broken off and, furthermore, inserted into the other end of the sleeve, the objective of positioning is achieved and, furthermore, ensures that the hypodermic needle is not exposed.