The present invention relates to a disposable safety syringe for completely moving the push rod forward after injection to be combined with the needle and then moving the push rod backward to isolate the needle in the cylinder, even if the push rod moves forward before injection. The present invention relates to a disposable safety syringe that is not combined with and can be used normally. The present invention provides a cylinder, a coupling tube protruding at the front end of the cylinder, a push rod inserted into the cylinder so as to be movable back and forth, a needle fixing tube coupled to the coupling tube, and a front and rear inside the needle fixing tube. An adapter tube is inserted to enable movement and the needle tube is pulled into the cylinder while the needle holder tube is caught when moving to the rear, and the tube is protruded to the front end of the rod to catch the adapter tube when the rod is moved forward. And an elastic ring configured to press the elastic catching portion and limit the forward movement of the push rod, and expand and contract when the push rod moves backward to allow the forward movement of the push rod； The elastic catching portion is formed along the outer circumferential surface of the projection and the first elastic dog ring is formed protruding on the outer peripheral surface of the protrusion to the front end of the push rod so as to penetrate the elastic ring, the hook with the outer tube is made； A first step interfering with the expansion ring in the compressed state； The inner circumferential surface of the adapter tube is provided with a second stepped jaw with the elastic towing ring； Both sides of the outer circumferential surface of the protrusion are formed at the rear of the first elastic dog ring is provided with an elastic extension protrusion for applying a continuous external force in the direction in which the expansion ring is extended while passing through the expansion ring when the push rod is mov
