Disclosed is that the first piston and the rotating part are gradually rotated in one direction in the groove of the second piston while sequentially passing the step of injecting the chemical solution into the cylinder of the syringe to allow the chemical solution to be injected into the body, and the injection of the chemical solution is completed to reuse. When the hazard piston is separated from the syringe, the second piston remains in the cylinder and only the first piston and the rotating part are separated from the second piston, and thus relates to a disposable syringe that cannot be reused. Disposable syringes that cannot be reused according to an embodiment of the disclosed subject matter have a cylindrical storage space in which a chemical solution is stored, a scale is printed on the outer circumferential surface, a tip into which a needle is inserted at the bottom, and an insertion hole at the top. A cylinder formed in communication with the cylinder, which is inserted into the storage space through the insertion hole, moves forward and backward in the vertical direction, and is provided with a pressing portion pressed by a user at the top, and a guide protrusion protruding in the longitudinal direction at the lower outer periphery. A piston, a rotatable portion rotatably coupled to a lower portion of the first piston, and a protruding portion protruding from its lower outer periphery, and an engaging portion into which a lower portion and a rotatable portion of the first piston are inserted, are formed inside the upper portion. 1 While the lower side and the rotating part of the piston are inserted into the storage space while being inserted into the insertion hole, the guide groove is formed on one side of the inner periphery of the coupling hole, and the other side of the inner periphery is moved. A groove portion to which the projection is moved is formed, and is integrally moved with the first piston and the rotating portion according to the seque
