PURPOSE: To prevent erroneous detection by estimating a threshold level corresponding to a true ambient temperature at the time of detecting a temperature during a self-heating time by means of an operation.

CONSTITUTION: The temperature inside a compressor is detected, converted into a threshold level by an operating part 8, and stored by a RAM 9. The temperature is detected at every defined time and a true temperature inside the compressor is estimated by means of arithmetic processing in a ROM 10, which is converted into a threshold level and compared with the value at the time of self-heating to judge the drying-up of oil. Hence, any erroneous operation due to change in the temperature inside a compressor cam be prevented.