To miniaturize a device with an efficient structure, reducing the cost, and improving COP (coefficient of performance) by utilizing the conversion of expansion energy of the working fluid into mechanical energy.
An expander 5 for converting expansion energy of the working fluid into mechanical energy, and a sub-compressor 6 driven by the expander are accommodated in one sealed container 4. The refrigerant discharged from a main compressor 2 is partially branched to flow into the container 4, and the pressure in the sealed container is kept at high pressure same as the discharge pressure. Whereby the viscosity of the lubricant is lowered, the sufficient amount of lubricant can be supplied to a sliding part of the expander and the sub-compressor by pressure difference, and the sealing performance can be improved. Further the sealed container 4 can be also used as a receiver tank for storing the liquid refrigerant.