A gas compressor includes a housing with suction and discharge ports, and a gas-compressing mechanism housed in the housing to compress a gas sucked through the suction port and discharge the compressed gas via the discharge port. The mechanism includes a cylinder defining a cylinder chamber with a circular or elliptical cross-sectional shape. The cylinder has a suction inlet and a discharge outlet. The suction inlet and the discharge outlet are opened in a peripheral wall of the cylinder chamber. A suction passage is formed along an outer periphery of the cylinder corresponding to a peripheral direction of the cylinder chamber. A discharge passage extends inside a thick portion of the cylinder along a center axis of the cylinder chamber. The suction inlet communicates with the suction port via the suction passage. The discharge outlet communicates with the discharge port via the discharge passage.