One of the safety analysis for superconducting magnet system in International Thermonuclear Experimental Reactor (ITER) was carried out. The ITER cryostat will hold many superconducting magnets, such as twenty of toroidal field coils, a central solenoid coil, and seven poloidal coils. Loss of vacuum of the cryostat was considered as the worst assumption and the safety analysis of the magnets was examined when the assumption would be occurred. Accordingly, the loss of vacuum will cause the loss of thermal shield vacuum for the magnets and then a large heat transfer will be generated in the cryostat The magnet pressure and temperature will rise, bringing the magnets to quench. Such behavior was simulated by using a developed computer-aided calculation code. As a result of the calculation, a catastrophic phenomenon doesn't appear in the assumption. It is observed that a quasi-stable state, where the magnet temperature is kept to be less than 7 K, is maintained for more than 600 seconds. Thus, the magnet current can be slowly discharged like as the ordinal operation without magnet quench even in such worst assumption due to a large volume of the cryostat.