To improve a period coefficient of performance by preventing over compression of a screw compressor with a simple structure and improving efficiency in an especially light load zone operation range.

In the screw compressor, a compression chamber is formed by a pair of screw rotor provided with a male rotor 2 and a female rotor 2A, and a casing holding a pair of the screw rotors. A delivery port 10 from which compressed gas flows out and a delivery chamber 12 in which compressed gas delivered from the delivery port flows, are formed in the casing. Bypass passages providing communication between the compression chamber and the delivery chamber are provided at both of the casing at the male rotor side and the female rotor side near the delivery port respectively. A valve 110 opening and closing the bypass passage is provided.