A screw compressor includes an economizer passage (50) provided in a casing (1) and allowing an outside of the casing (1) and a slide groove (14) in which a slide valve (12) is provided to communicate with each other, and an economizer port (12p) provided in the slide valve (12) and allowing the economizer passage (50) to communicate with one of compression chambers (11) in accordance with a position of the slide valve (12). The slide valve (12) gradually advances a timing of starting discharge by moving from a discharge side toward a suction side. The economizer port (12p) is provided at a position where the economizer port (12p) communicates with a suction-pressure chamber (1C) when the slide valve (12) is positioned at an extreme end on the suction side.
