To provide a gas compressor capable of properly advancing a vane even in starting, and having a fail-safe function for an excessive compression state of a suction compression chamber.

In this gas compressor 10, a compressor body 12 stored inside a housing 11 has a rotor 20, a cylinder body 16 and the vane 23 held by a slit groove 24 of the rotor 20, and the suction compression space 25 is defined. A guide groove 54 is arranged in at least one 18c of an inner wall surface of a cylinder 16, and the vane 23 is provided with an engaging part 50 freely going in and out of a side wall surface 23c to be engageable with the guide groove 54. The guide groove 54 presents an annular ring shape along a locus drawn by a fixed point of the vane 23 advancing-retreating from the slit groove 24 when the tip 23a follows a contour shape of the cylinder 16, and is extended until the suction compression space 25 defined on the front side of the vane 23 when viewed in the rotational direction of the rotor 20, reaches a predetermined position in a compression stroke from a suction stroke.