A unit incorporating a reservoir tank and an expansion valve into a single built-in structure comprises a reservoir tank 10 sealed by a cover 20, and an expansion valve 30 fixed on the cover 20. A refrigerant pipe 40 extending through the cover 20 introduces a refrigerant from a compressor through a condenser into the reservoir tank 10. The refrigerant introduced into the expansion valve 30 through a pipe 50 increases in volume, and then enters an evaporator. After passing the evaporator, the refrigerant in the gaseous phase flows through pipe 62 into the expansion valve 30, then delivers thermal information of the refrigerant to a heat sensitive member of the expansion valve, and travels in a pipe 70 extending into and from the reservoir tank 10. While traveling along the pipe 70, the gaseous refrigerant exchanges heat with the liquid refrigerant stored in the reservoir tank, and then returns to the compressor.