To provide a system for treating a used syringe, which is simple, inexpensive and safe.

(a) A plastic cap (3) is fitted to the tip of an injection needle (2) of a syringe body (1) in consideration of safety using a tool or the like, and after that, a metal cap (4) with a slot (8) is put on the plastic cap (3) to adjust the dimensions of an opening at the open end side with one end closed. (b) The used syringe (6) subjected to the treatment (a) is stored in a storing bag (5) made of a thin material such as a metal foil of an aluminum or the like and opened only at one end, and the opening is closed. (c) The used syringe (6) put in the state of (b) is stored in the storing bag (5), and installed in a heater (7), and after heating and heat insulation under specified conditions, cooling is performed and the storing bag is taken out.