PURPOSE: To make it possible to positively carry out the operation of the defrosting control unit by detecting the temperature on the inlet side of a pipeline of an indoor side heat exchanger and the temperature at the central portion thereof.
CONSTITUTION: Pipeline temperature detecting elements 6, 6' are provided in an inlet pipeline of an indoor side heat exchanger 3 and almost at the center, respectively. When an outdoor heat exchanger 5 reaches a frosted state, an inlet pipeline temperature t 1 is extremely lowered, and simultaneously a central portion pipeline temperatures t 2 is lowered. That is, if a temperature difference between the inlet pipeline temperature t 1 and the central portion pipeline temperature t 2 becomes less than a set pipeline temperature t, it is necessary to defrost. Furthermore, depending upon the power source frequency, the ability of the compressor 1 is varied and also the high pressure and the exhaust temperature at the time of the frosting of the outdoor heat exchanger 5 are varied. That is, since the inlet side pipeline temperature t 1 of the indoor side heat exchanger 3 is varied, the defrosting judgement is carried out by switching the set temperature (t) to 50Hz and 60Hz.