PURPOSE: To perform a direct following-up against an operating condition of an air conditioner or a rapid variation in environment of use of the air conditioner and adjust a degree of opening of an expansion valve by a method wherein an objective degree of opening of the expansion valve is determined based on a return air temperature, an outer air temperature and an output of a compressor.

CONSTITUTION: A return air temperature sensor 13 acting as a return air temperature selecting means and a compressor controller 14 for use in controlling an output P of a compressor 2 are connected to a controller H, an invertor frequency (f) is adjusted in such a manner that a return air temperature (tr) may approach an objective return air temperature (trm) based on detected information of the return air temperature sensor 13 and there is provided a compressor output adjusting means 101 for use in adjusting the temperature adjusting capability of a heat exchanger 1A acting as an evaporator. In addition, an outer air temperature sensor 15 acting as an outer air temperature sensing means and an expansion valve controller 4a for use in operating to open or close the electrical expansion valve 4 are connected to the controller H, thereby an objective degree of opening Sm of the thermal expansion valve 4 is calculated based on the return air temperature (tr), the outer air temperature (to) and the compressor frequency (f) and there is provided a valve controlling means 102 for adjusting the degree of opening of the expansion valve S to the objective degree of opening Sm.