A screw compressor includes compressor bodies, a motor disposed in a side of the compressor bodies, a gear box coupled to the compressor bodies and transmitting the drive force of the motor to a screw rotor, and a tubular coupling casing coupling the gear box and the motor and having horizontal axis. The coupling casing has a work hole in the horizontal direction. The work hole is used for maintaining a coupling accommodated in the coupling casing. The coupling casing has a rib extending in the up-down direction. It is thus possible to avoid reduction in the rigidity of the coupling casing with no cost increase, and to improve maintainability.