A compressor unit, comprising: a casing; a pair of male and female screw rotors contained in the casing and meshing with each other; a compressor having the casing and the screw rotors and for compressing sucked gas and discharging it; a balance cylinder formed in the casing; a balance piston for pressing a rotor shaft acting as a rotating shaft of at least one of the screw rotors in an axial direction by a fluid pressure, wherein the balance piston is integrally mounted to the shaft of one screw rotor and fitted in the balance cylinder; an oil supply passage for supplying oil to a pressure chamber; a capacity adjuster for adjusting capacity of the compressor; and a balance piston control device for adjusting pressure of a fluid applied on the balance piston according to the capacity of the compressor, characterized in that the oil supply passage supplies oil to a high pressure chamber closer to the screw rotors than the balance piston an oil supply pressure detector for detecting pressure (Pb) of pressure oil supplied to the balance cylinder and an opening-adjustable control valve located upstream of the oil supply pressure detector are provided in the oil supply passage, and a low pressure chamber within the balance cylinder opposite to a portion facing a high pressure-side of the balance cylinder communicates with a low-pressure compression space close to the suction passage within the rotor chamber through a low-pressure communicating passage, wherein the difference between the internal pressure of the high-pressure chamber and the internal pressure of the low pressure chamber generates a force pressing the balance piston in an axial direction of the rotor shaft toward the low-pressure chamber from the high-pressure chamber. a casing; a pair of male and female screw rotors contained in the casing and meshing with each other; a compressor having the casing and the screw rotors and for compressing sucked gas and discharging it; a balance cylinder formed in the casing; a balance piston for pressing a rotor shaft acting as a rotating shaft of at least one of the screw rotors in an axial direction by a fluid pressure, wherein the balance piston is integrally mounted to the shaft of one screw rotor and fitted in the balance cylinder; an oil supply passage for supplying oil to a pressure chamber; a capacity adjuster for adjusting capacity of the compressor; and a balance piston control device for adjusting pressure of a fluid applied on the balance piston according to the capacity of the compressor, characterized in that the oil supply passage supplies oil to a high pressure chamber closer to the screw rotors than the balance piston an oil supply pressure detector for detecting pressure (Pb) of pressure oil supplied to the balance cylinder and an opening-adjustable control valve located upstream of the oil supply pressure detector are provided in the oil supply passage, and a low pressure chamber within the balance cylinder opposite to a portion facing a high pressure-side of the balance cylinder communicates with a low-pressure compression space close to the suction passage within the rotor chamber through a low-pressure communicating passage, wherein the difference between the internal pressure of the high-pressure chamber and the internal pressure of the low pressure chamber generates a force pressing the balance piston in an axial direction of the rotor shaft toward the low-pressure chamber from the high-pressure chamber.