In a compression mechanism (30) of a rotary compressor, cylinder chambers (60, 65) are formed by a cylinder (40) and a second housing (50). A back side space (75) is formed between an end plate (41) of the cylinder (40) and a flat plate section (36) of a first housing (35). A communication path (81) and a differential pressure valve (82) are arranged in the first housing (35). When the differencebetween a discharge pressure and a suction pressure is small, the discharge pressure is introduced into an intermediate space (77) through the communication path (81), which causes both an inner space (76) and the intermediate space (77) to be at the discharge pressure. As a result, pressing force acting on the cylinder (40) is greater. On the contrary, when the difference between the discharge pressure and the suction pressure is large, the communication path (81) is blocked by the differential pressure valve (82), which causes the pressure in the intermediate space (77) to be at an intermediate pressure lower than the discharge pressure. As a result, pressing force acting on the cylinder (40) is smaller.