An object of this invention is to prevent drop of life of a bearing by an axial force when using a pump as a compressor. In a vacuum pump 1 compressing and discharging gas in a direction of a rotor axis by rotation of screw rotors 3, 4 engaged together which are supported rotatably in a casing 2, balance pistons 13, 14 are disposed on shafts 6, 7 of said screw rotors at inlet side of said casing. The balance pistons separate a receiving section 17 at area of the screw rotor and a pressurizing section 16 at area of the balance piston, and a thrust force of the screw rotors at a pressurizing condition is canceled by acting the discharge pressure in the pressurizing section. The pump is used as a compressor when the discharge pressure is acted on the balance pistons 13, 14. When the pump is used as a vacuum pump, air at discharge side is sucked as cool air through a cooler toward a place near to the discharge side of the receiving section 17 at area of the screw rotor.