To provide a gas compressor having a case not to depend on the specifications of a gas compressor, and to reduce cost, facilitate casting and machining, and reduce incurring of a loss due to a casting defect and a machining miss.

A compressor body is securely inserted in a case, and the case is divided into a front casing 91 independent corresponding to the number of the compressor bodies, and a rear casing 53 having a discharge chamber and a discharge port. The front casing 91 is provided with an inner wall machined with high precision so that airtightness between the compressor body and the front casing is kept. Meanwhile, a rear casing 53 does not demand for special precision. Flanges 93A and 93B capable of being coupled with each other are disposed at the division end part of the rear casing 53 and the division end part of the front casing 91.