The object of the present invention is to prevent the second blade of a compression system with a muti-cylinder rotary compressor from generating conflicting sound. The compression system comprises a multicylinder rotary compressor being configured to be used by switching of a first operation mode in which both rotary compression elements (32,34) perform a compression work and a the second operation mode in which the only first rotary compression element (32) substantially performs the compression work. When the second operation mode is switched to the first operation mode, discharge-side pressures of both the rotary compression elements (32,34) are applied as a back pressure of the second vane (52), and thereafter an intermediate pressure is applied which is between suction-side and discharge-side pressures of both the rotary compression elements. When the first operation mode is switched to the second operation mode, a valve device (106) interrupts flowing of a refrigerant into a second cylinder (40), and thereafter suction-side pressures of both the rotary compression elements (32,34) are applied as the back pressure of the second vane (52).