A capacity control device for a screw compressor provided with a capacity control passage and a slide valve adjusting the opening thereof, wherein the slide valve is driven in a closing direction due to a pressure difference between a high pressure and a low pressure and is moved in an opening direction thanks to function of a spring when the high pressure is balanced with the low pressure, and when the pressure difference between the high pressure and low pressure is lower to cause the slide valve to be open thanks to the function of the spring, the slide valve is moved in the closing direction against the spring by use of pressure in a compression-processing part in the screw compressor, thereby enabling to quickly transit from no load condition to a loaded condition.