A screw compressor comprising: - a casing having a discharge port and including a cylindrical inner cylinder-surface portion; - a screw rotor housed in the inner cylinder-surface portion of the casing, being rotatable, and having a plurality of screw grooves on an outer periphery thereof; - a gate rotor having tooth portions that are provided on an outer periphery and come into mesh with the screw grooves, the gate rotor defining compression chambers in combination with the screw grooves and the inner cylinder-surface portion; - a suction-pressure chamber provided in the casing and having a suction-pressure atmosphere; - a slide groove provided on the inner cylinder-surface portion of the casing and extending in a direction of a rotating shaft of the screw rotor; - a slide valve (12, 120) provided in the slide groove and being slidable in the direction of the rotating shaft of the screw rotor, and the slide valve (12, 120) configured to adjust a timing of starting discharge; - an economizer passage provided in the casing and allowing an outside of the casing and the slide groove to communicate with each other; and - an economizer port (12p, 120p) provided in the slide valve (12, 120) and allowing the economizer passage to communicate with one of the compression chambers in accordance with a position of the slide valve (12, 120), - wherein the slide valve (12, 120) gradually advances the timing of starting discharge as the slide valve (12, 120) proceeds toward a suction side from a discharge side, and - characterized in that the economizer port (12p, 120p) is provided at a position where the economizer port (12p, 120p) communicates with the suction-pressure chamber when the slide valve (12, 120) is positioned at an extreme end on the suction side. - a casing having a discharge port and including a cylindrical inner cylinder-surface portion; - a screw rotor housed in the inner cylinder-surface portion of the casing, being rotatable, and having a plurality of screw grooves on an outer periphery thereof; - a gate rotor having tooth portions that are provided on an outer periphery and come into mesh with the screw grooves, the gate rotor defining compression chambers in combination with the screw grooves and the inner cylinder-surface portion; - a suction-pressure chamber provided in the casing and having a suction-pressure atmosphere; - a slide groove provided on the inner cylinder-surface portion of the casing and extending in a direction of a rotating shaft of the screw rotor; - a slide valve (12, 120) provided in the slide groove and being slidable in the direction of the rotating shaft of the screw rotor, and the slide valve (12, 120) configured to adjust a timing of starting discharge; - an economizer passage provided in the casing and allowing an outside of the casing and the slide groove to communicate with each other; and - an economizer port (12p, 120p) provided in the slide valve (12, 120) and allowing the economizer passage to communicate with one of the compression chambers in accordance with a position of the slide valve (12, 120), - wherein the slide valve (12, 120) gradually advances the timing of starting discharge as the slide valve (12, 120) proceeds toward a suction side from a discharge side, and - characterized in that the economizer port (12p, 120p) is provided at a position where the economizer port (12p, 120p) communicates with the suction-pressure chamber when the slide valve (12, 120) is positioned at an extreme end on the suction side.