An overlock sewing machine which can switch its operation from an ordinary over-edge chain stitch to a rolled seam or vice versa with no requirement for a complicated operation of adjusting thread tension is obtained. This overlock sewing machine is provided with a spring (42) for providing a thread with prescribed tension. The overlock sewing machine is further provided with a switching mechanism (41, 43, 44) for switching the thread tension between that for the rolled seam and that for the over-edge chain stitch by switching the spring between a tensioning state and a non-tensioning state. Thus, the thread tension is readily switched between that for the rolled seam and that for the over-edge chain stitch.