PURPOSE: To make it possible to positively conduct defrosting operation even when a limit switch is opened or closed by an erroneous operation and the like of said switch by preventing a power source to an electronic control circuit for defrosting from being disconnected by a switch other than a power source switch.

CONSTITUTION: Even when a limit switch 4 is made open, if a power switch 6 is closed, power is being supplied to an electronic control circuit 5. When the electronic control circuit 5 detects the temperature by a sensor 9 fitted to an evaporator 12 while integrating an operational time value and such conditions as 1hr operation and the temperature of 0°C or less being detected by the sensor 9 are satisfied, the humidifier enters into a defrosting operation, a compressor 1 and a blower 2 are stopped, and power is supplied to a heater 3. Further, during the defrosting operation, the sensor 9 conducts the temperature detection, and when the temperature reaches 12°C or more, it is judged that frost is completely removed, and the mode of operation returns to an ordinary dehumidifying operation. Further, upon this occasion, the timer is reset, and the values are newly integrated. Power is continuously supplied to the electronic control circuit regardless of the operation of the limit switch, and defrosting is positively carried out without the timing of defrosting being jumped.