outdoor heat exchanger 1 via a capillary tube 3 in outdoor heat exchanger 1 the refrigerant is changed into gas of low pressure by absorbing heat from outside atmosphere if the heat absorption is not efficiently carried out due to frosting of outdoor heat exchanger 1 the temperature of outdoor heat exchanger 1 cannot be lower than a predetermined temperature this can be sensed by temperature sensor 1 39 attached to outdoor heat exchanger 1 which sensor then supplies a frost signal to microprocessor 12 steps 502 and 503 on the other hand the refrigerant which absorbed heat from outdoor heat exchanger 1 is introduced into r side of a four way reversing valve 4 when receiving an excess frost signal from temperature sensor 1 39 as mentioned above point t1 in fig3 microprocessor 12 actuates timer 11 to count time step 506 after a predetermined time elapses point t1 in fig3 microprocessor 12 operates such that invertor 8 outputs an ac voltage of low frequency to compressor 5 steps 507 and 508 accordingly compressor 5 drives at low speed so that the difference between discharge pressure and suction pressure in the compressor is reduced to pl refer to fig3 when the time of timer 1 1 reaches t2 steps 509 and 510 microprocessor 12 sends a signal to a four way reversing valve 4 to connect r side and p.