PURPOSE: To economize energy, by controlling the quantity of supplied cooling water to an intermediate cooler to maintain the discharge pressure of a compressor at a prescribed level.

CONSTITUTION: A discharge pressure gauge 18 is connected to a controller 20. Actual discharge pressure is compared with set pressure which is set by a setting unit 21. The degree of opening of cooling water control valves 12, 13, 14 is adjusted to reduce the difference between the compared quantities. The discharge pressure of a compressor is thus maintained at a prescribed level. This results in reducing shaft power and economizing energy.