A screw compressor having a male rotor and a female rotor that have rotation axes substantially parallel to each other and rotate while meshing with each other; a main casing that has a bore for housing the male rotor and the female rotor; a discharge casing that is connected to the rotor axis direction discharge side of the main casing and provided with a discharge side end face which abuts on the end face of the main casing to cover the opening of the bore; a discharge chamber or a discharge flow path that discharges a compressed gas from a compression operation chamber (36A, 36B) formed by the male rotor and the female rotor through a discharge port (23A, 23B; 25A, 25B) formed in at least either the main casing or the discharge casing; a valve hole that is disposed on the female rotor side of the discharge port (23A, 23B; 25A, 25B), formed in the discharge side end face of the discharge casing toward at least either the male rotor or the female rotor, and open to the compression operation chamber (36A, 36B); a bypass flow path that establishes communication between the valve hole and the discharge chamber or the discharge flow path; and a valve disc that is disposed in the valve hole; the screw compressor comprising: a valve disc drive device that opens and closes the valve disc; and a control device that detects whether the compression operation chamber (36A, 36B) is over-compressed, and if the compression operation chamber (36A, 36B) is over-compressed, controls the valve disc drive device so as to open the valve disc, characterized in that the valve disc drive device includes a cylinder that is mounted on the rear side of the valve disc, a piston that reciprocates in the cylinder, and a rod that connects the piston to the valve disc, a cylinder on the valve disc side of the piston is provided with a spring that presses the piston to the side away from the valve disc so that a compressed gas on the discharge side of the screw compressor is introduced into a cylinder on the side away from the valve disc of the piston; and a path with a capillary tube is used to connect the valve disc side cylinder of the piston to the discharge side of the screw compressor; wherein a continuous path is provided to establish communication between the cylinder side of the path with the capillary tube and a low-pressure space of the screw compressor; wherein a solenoid valve is installed in the middle of the continuous path to open and close the continuous path; and wherein the pressure on the discharge side of the screw compressor is applied into the cylinder on the valve disc side of the piston to open the valve disc by opening the continuous path when no over-compression has occurred and by closing the continuous path when over-compression has occurred. a valve disc drive device that opens and closes the valve disc; and a control device that detects whether the compression operation chamber (36A, 36B) is over-compressed, and if the compression operation chamber (36A, 36B) is over-compressed, controls the valve disc drive device so as to open the valve disc, characterized in that the valve disc drive device includes a cylinder that is mounted on the rear side of the valve disc, a piston that reciprocates in the cylinder, and a rod that connects the piston to the valve disc, a cylinder on the valve disc side of the piston is provided with a spring that presses the piston to the side away from the valve disc so that a compressed gas on the discharge side of the screw compressor is introduced into a cylinder on the side away from the valve disc of the piston; and a path with a capillary tube is used to connect the valve disc side cylinder of the piston to the discharge side of the screw compressor; wherein a continuous path is provided to establish communication between the cylinder side of the path with the capillary tube and a low-pressure space of the screw compressor; wherein a solenoid valve is installed in the middle of the continuous path to open and close the continuous path; and wherein the pressure on the discharge side of the screw compressor is applied into the cylinder on the valve disc side of the piston to open the valve disc by opening the continuous path when no over-compression has occurred and by closing the continuous path when over-compression has occurred.