An apparatus for changing capacity of a multi-stage compressor comprises: a first cylinder provided with a first suction and a first discharge port, and divided into a first suction chamber and a first compression chamber by a first rolling piston which orbits and a first vane which makes a linear movement in contact with the first rolling piston; a second cylinder provided with a second suction port and a second discharge port, and divided into a second suction chamber and a second compression chamber by a second rolling piston which orbits and a second vane which makes a linear movement in contact with the second rolling piston; a middle bearing inserted between the first cylinder and the second cylinder, having a bypass hole to allow communication between the compression chambers of the first cylinder and the second cylinder, and having a valve hole for communication in the middle of the bypass hole; a sliding valve slidingly coupled to the valve hole of the middle bearing, and selectively opening or closing the bypass hole; and a pressure switching unit for selectively supplying discharge pressure to one side of the sliding valve, thereby changing a capacity using all the plural compression units, and obtaining power saving effect suitable for the saving mode.