To reduce the oil outflow amount to a plate driving mechanism for rotationally driving a control plate for changing the compression capacity.

A gas compressor is provided with a rotor rotatably provided in a cylinder chamber having the elliptic cylindrical inner periphery, a vane provided in a vane groove provided on the rotor so as to be freely advanced and retracted, a control plate for controlling the capacity of gas to be sucked to the cylinder chamber by the rotation of the rotor, and a plate driving mechanism for rotationally driving the control plate, and the gas compressor compresses sucked gas and then discharges it to a discharge chamber. The plate driving mechanism 60 is provided with a driving piston 30 for rotating the control plate 12, a communication passage 32 for communicating a control pressure chamber 26 of the driving piston with the discharge chamber, and an outflow shut-off part for shutting off the outflow of oil from the control pressure chamber to the discharge chamber through the outer periphery of the driving piston.