PURPOSE: To improve control characteristic generally over low load section, by performing rotation control of internal-combustion engine coupled with compressor and volume capacity of compressor for reducing variation of load.

CONSTITUTION: In interval C where rotation of engine is from rated level to cross point A, a slide valve will move to the direction for reducting partial load capacity as reduction of engine rotation within the range where the engine will never be overloaded for reducing variation of load. Thereafter compressor is operated only through capacity control, thereby control characteristic can be improved not only for the portion near full load capacity but for low load portion.