A vane compressor has a vane back pressure adjustment device including back-pressure chambers defined in a vane-supporting rotor and surrounded by vanes and side blocks secured to axial ends of a cylinder. The back-pressure chamber has axial ends disposed to follow a path against each of the side blocks when the rotor rotates. The path is divided into at least one high-pressure zone in which the vanes move across a pump outlet defined in the cylinder, and at least one normal-pressure zone which is the remainder of the path. In the normal-pressure zone, oil grooves defined in surfaces of the side blocks which face the rotor and supplied with oil from an oil sump in the vane compressor are in communication with the back-pressure chambers, to thereby maintain the oil pressure in the back-pressure chambers at a pressure level in the oil sump. In the high-pressure zone, the back-pressure chambers are out of communication with the oil grooves so that the oil fed into the back-pressure chambers is confined therein, except through a restrictor means. When the vanes move across the pump outlet, the vanes enter the high-pressure zone to trap oil in the back-pressure chambers to thereby elevate the oil pressure in the back-pressure chambers to a high level. Therefore, the vanes are prevented from being retracted deeply into vane slits in the rotor and then popping out into hitting engagement with the cylinder.