A screw compressor comprising: a casing in which a discharge chamber and a suction chamber are provided; a screw rotor, on an outer peripheral surface of which a plurality of grooves forming a compression chamber are provided, the screw rotor being rotatable in the casing; a variable Vi valve movable in an axial direction of the screw rotor and whose stopping position is shifted to change an internal volume ratio that is a ratio between a volume of the compression chamber at completion of suction and a volume of the compression chamber at completion of discharge; a driving device including a Vi piston coupled to the variable Vi valve, and a driving cylinder that houses the Vi piston movably in the driving cylinder and inside of which is divided by the Vi piston into two cylinder chambers; a pressure-switching mechanism that switches pressure to be introduced into the driving device, wherein the pressure-switching mechanism includes a solenoid valve configured to open to form a first passage through which the discharge chamber and a pressure-introducing hole provided at the one cylinder chamber communicate with each other, and close to block the first passage, the solenoid valve being switched to open to enable introduction of discharge pressure from the discharge chamber into the one cylinder chamber or close to disable introduction of discharge pressure from the discharge chamber into the one cylinder chamber, and wherein the pressure in the one cylinder chamber changes with opening or closing of the solenoid valve; characterized by an expansion mechanism provided at a passage communicating between a pressure-releasing hole and the suction chamber, the pressure-releasing hole being provided at one of the two cylinder chambers; wherein the driving device moves the Vi piston by changing pressure in the one cylinder chamber to control a position of the variable Vi valve. a casing in which a discharge chamber and a suction chamber are provided; a screw rotor, on an outer peripheral surface of which a plurality of grooves forming a compression chamber are provided, the screw rotor being rotatable in the casing; a variable Vi valve movable in an axial direction of the screw rotor and whose stopping position is shifted to change an internal volume ratio that is a ratio between a volume of the compression chamber at completion of suction and a volume of the compression chamber at completion of discharge; a driving device including a Vi piston coupled to the variable Vi valve, and a driving cylinder that houses the Vi piston movably in the driving cylinder and inside of which is divided by the Vi piston into two cylinder chambers; a pressure-switching mechanism that switches pressure to be introduced into the driving device, wherein the pressure-switching mechanism includes a solenoid valve configured to open to form a first passage through which the discharge chamber and a pressure-introducing hole provided at the one cylinder chamber communicate with each other, and close to block the first passage, the solenoid valve being switched to open to enable introduction of discharge pressure from the discharge chamber into the one cylinder chamber or close to disable introduction of discharge pressure from the discharge chamber into the one cylinder chamber, and wherein the pressure in the one cylinder chamber changes with opening or closing of the solenoid valve; characterized by an expansion mechanism provided at a passage communicating between a pressure-releasing hole and the suction chamber, the pressure-releasing hole being provided at one of the two cylinder chambers; wherein the driving device moves the Vi piston by changing pressure in the one cylinder chamber to control a position of the variable Vi valve.