Abstract: Disclosed herein is a safety syringe. According to the safety syringe of the present invention, an adapter connected to an injection needle is threadedly coupled with the neck of a cylinder while interposing an O-ring therebetween. Thus, the movement of the injection needle is effectively prevented while in use, and thus the injection needle can be pricked accurately at a desired position of the patient's body. Since the adapter is formed with grooves, whereas a plunger is formed with two protrusions positioned diametrically opposite to each other so as to be inserted into the grooves, the adapter is rotated and then moved downwardly in accordance with the successive rotation and downward movements of the plunger after the adapter is released from the neck of the cylinder. The plunger is formed at its upper surface with a vertical rod extending upwardly from an eccentric position, whereas the adapter is defined with a conical cavity.