A scroll compressor having injection ports 70A and 70B opened to operating chambers Vc during a predetermined period of a complete compression cycle, so that a part of the refrigerant circulating in a refrigerating system is injected into the compression chambers from the injection ports. The injection ports are opened to an intermediate pressure chamber, which is at a pressure between the pressure of the refrigerant sucked into the compressor and the pressure of the refrigerant discharged from the compressor. The intermediate pressure chamber is arranged so as to eliminate pressure pulsations therein. Check valves 72A and 72B are provided in an end plate 54 of a stationary scroll member 50. Furthermore, relief valves may also be provided in the end plate for releasing the pressure of the operating chambers to a low pressure side of the refrigerating system.