PURPOSE: To restrain excessive temperature rise due to defrosting operation upon detecting disconnection by a method wherein the time of defrosting operation is switched forcibly into an actual defrosting time, memorized before detecting the disconnection, after detecting the disconnection of a temperature sensor, detecting the finishing of the defrosting operation.

CONSTITUTION: A controller 15, controlling the start and the finish of the defrosting operation of a defrosting heater 12, stores the count time of a counting means 13 every time when the operation of the defrosting heater 12 is finished and when the controller 15 has received an abnormal signal from a disconnection detecting device 20, the controller 15 finishes the defrosting operation of the defrosting heater 12 and energizes the defrosting heater 12 by only the stored count time from the defrosting operation of next time.