To quickly defrost freezing caused in a safety valve and prevent freezing caused in the safety valve by providing a heating means for heating the safety valve in the safety valve used in a cryostat or the like.

When the detected temperature of a temperature sensor 12 is lower than the set temperature, a temperature controller 17 turns on a switch 16. Therefore, a current is carried to a heater 11 so that the heater 11 heats a heating block 14. The heat is transmitted from the heating block 14 to the body of a safety valve 13. Normally, an O-ring fitted to a poppet 43 of the safety valve 13 is pressed to the body by a spring, so that the heat of the body is transmitted to the O-ring to be heated. When the detected temperature of the temperature sensor 12 reaches the set temperature, the temperature controller 17 turns off the switch 16. Therefore, current carrying to the heater 11 is stopped so the heating of the heating block 14 is ended. Thus, the temperature of the safety valve 13 is always kept to a fixed temperature or more so as to surely prevent adhesion of dew and frost.