A rotary compressor for compressing a refrigerant gas has a by-pass port and a control valve means. The by-pass port communicates at one end with a suction chamber containing the gas to be compressed, and at the other end with a compression chamber wherein compression of the refrigerant gas is in progress. The control valve means includes a valve spool accommodated in a chamber formed in a front side plate of the compressor and movable between its full-load position, where a gas suction port is opened to the compression chamber while the by-pass port is closed, and its partial-load position where the former is at least partially closed while the latter is opened. The valve spool is normally biased to its partial-load position. During operation of the compressor, the valve spool is placed under the influence of a first pressure of the gas acting to urge the same towards its said full-load position, and to a second pressure of the gas to urge the same towards its said partial-load position. The spool is shifted automatically to its partial-load position when the difference between these first and second pressures is decreased with a drop of cooling load. The compressor of the invention further comprises a flow control means for regulating the flow of the gas into its compression chambers in the event of a rapid increase of compressor speed.