A screw compressor includes a valve hole formed at a discharge side end surface of a discharge casing and at a position opening to a compression work chamber; a bypass flow path having the valve hole and a discharge chamber communicate with each other; and a valve body arranged in the valve hole. The screw compressor also includes cylinder chambers provided on a rear surface side of the valve body; a piston reciprocally moving in the cylinder chambers; a rod connecting the piston and the valve body; communication paths for introducing a fluid on a discharge side into the cylinder chamber on a side opposite to a valve body side of the piston and on the valve body side; a pressure discharge path; a plurality of valve means; and a controller controlling the plurality of valves means.