A scroll back pressure chamber in a scroll compressor is composed of a space 111 filled with suction pressure (or intermediate pressure) and a space filled with discharge pressure, and the summation of the suction pressure (or the intermediate pressure) and the discharge pressure presses one of scrolls against the other of the scrolls. An injection hole, through which refrigerant in gas state or refrigerant in liquid state is injected into a compression chamber of the scroll compressor, is provided on a fixed scroll, and the gas refrigerant injection or the liquid refrigerant injection is selected and carried out according to an operating pressure ratio.