PURPOSE: To eliminate unnecessary defrosting operations by detecting the optimum defrosting timing by a combination of the elapsed time from the start of operation or from the completion of the last defrosting, the temperature of the outside unit heat exchanger, the lack or presence of the repetition of the switching on and off of a compressor and the outside air temperature and humidity conditions.

CONSTITUTION: In an outside air temperature sensor 10' and an outside unit electronic control device 25', a circuit A' to detect the temperature of the outside unit heat exchanger, a circuit B' to detect the elapsed time, a circuit C' to detect the outside temperature and humidity, and a circuit E' to detect the lack or presence of the switching on and off of a compressor are provided. The signal from the respective circuits A', B', C', E' is gathered by a defrosting timing decision output circuit F'. Considering the four elements of the elapsed time from the start of operation of the air conditioner or from the completion of the last defrosting, the refrigerant temperature of the outside unit heat exchanger, the lack or presence of the repetition of the switching on and off of the compressor and outside air temperature and humidity, the optimum defrosting timing of the air conditioner is detected by the optimum combination of those four or any three elements.