PURPOSE: To realize a maximum utilization of the output of an engine by slightly lowering the output rate of the engine by decreasing the amount of circulating refrigerant so as to reduce the engine output rate requirement of a compressor when the current output rate of the engine is found to be equal to the possible maximum engine output rate by the comparison of the two.

CONSTITUTION: A tentative output coefficient is determined in order to obtain current output characteristics of an engine when the current output rate of the engine is found to be equal to the maximum output rate of the engine by judging the current output rate of the engine. If the output coefficient of the engine is found to be equal to the tentative output coefficient, the engine is supposed to be operated at the maximum output rate; therefore, refrigerant load control is performed so that the engine output rate requirement of a compressor 2 is controlled not to exceed the maximum output rate. The simplest way to reduce the amount of the circulating refrigerant is to throttle an expansion valve 6 to thereby decrease the output rate requirement of the compressor for the engine 1; thus, the utilization of the engine output is maximized.