In a compression mechanism of a rotary compressor, a cylinder chamber is defined by a cylinder and a second housing. A back surface side gap is defined between an end plate part of the cylinder and a flat plate part of a first housing. The first housing is provided with a communicating path and a differential pressure regulating valve. When the difference between the suction pressure and the discharge pressure is small, the discharge pressure is introduced through the communicating path to an intermediate gap whereby both an internal gap and the intermediate gap are placed at the same pressure as the discharge pressure. Conversely, when the difference between the discharge pressure and the suction pressure is great, the communicating path is made discontinuous by the differential pressure regulating valve whereby the intermediate gap is placed at an intermediate pressure lower than the discharge pressure.