PURPOSE: To operate a two-stage compressor at a high speed by providing an oil separator in the middle of the air passage of an intermediate casing, providing an oil pump to recover the separated oil into an oil tank, and recovering the oil fed on the first stage side at the intermediate stage without bringing it to the second stage side.

CONSTITUTION: An oil separator 3 is provided at the intermediate stage between the first-stage compressor 1 and the second-stage compressor 2, the oil fed to the first-stage compressor 1 is separated, and the separated oil is boosted by an oil pump 4 and recovered into an oil tank 13. The reduction of the air temperature of the intake chamber on the second stage side and the mixing of the second-stage cooling oil and the first-stage cooling oil can be avoided, the compression process of the second-stage compressor 2 can be made isothermal compression, and high efficiency is obtained.