To help prevent global warming, the refrigerant HFC134a currently used in automobile air-conditioners has been slated for eventual elimination. CO₂, a natural refrigerant, has attracted attention as an alternative. We developed high-pressure control to improve evaporator, gas-cooler, intercooler, and compressor efficiency and to maximize COP (Coefficient Of Performance) in automobile airconditioning using a supercritical refrigeration cycle with CO₂ as the refrigerant. An evaluation of the system verified that these improved components provide equal or better COP than systems using HFC134a.