PURPOSE: To perform proper defrosting and improve the heating efficiency by a method wherein two or more temperature sensors are installed at two or more places in an outdoor heat exchanger and defrosting operation of the outdoor heat exchanger is carried out corresponding to a lower one among the detected temperatures of the sensors.

CONSTITUTION: The temperature of an outdoor heat exchanger 9 is monitored by temperature sensors 14 and 15 during heating operation, and when a lower one among the detected temperatures drops below a set point, time count is started. When time count reaches a set value, defrosting operation of the outdoor heat exchanger 9 is started. During defrosting operation, a four-way valve 8 is returned to a neutral state, and high temperature refrigerant of compressors 3, 4, 5 and 6 is directly supplied to the outdoor heat exchanger 9. When both the detected temperatures of the temperature sensors 14 and 15 rise up to a set point or higher, heating operation takes place again. Thereby, even if the outdoor heat exchanger 9 is divided into two sections, or it is a large model, frosting can be properly detected.