In a capacity-adjustable screw compressor unit, a thrust force caused by a reaction force of gas can be appropriately cancelled by a balance piston. A screw compressor unit of the present invention includes a compressor, having a pair of male and female screw rotors meshing with each other contained in a casing, for compressing sucked gas and discharging it, 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, a slide valve 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 calculated from a position of the slide valve.