An overlock sewing machine which can readily form a rolled seam with no requirement for a complicated tension adjusting operation is disclosed. In this overlock sewing machine, an auxiliary thread tensioner (15) is provided independently of a main thread tensioner (4), providing a lower looper thread (9) with thread tension which is necessary for forming an over-edge chain stitch, in the vicinity thereof. An end cam 25 is provided on a shaft 13 of a dial 12 for feed adjustment, so that an upper pawl 23 rams one portion of the peripheral edge of a tension disc 20 and expands a side which is opposed thereto for canceling tension provision to the lower looper thread (9) passed through tension discs and provides the tension only by the main thread tensioner (4) to carry out an ordinary over-edge chain stitch when a lower pawl 24 is engaged with a high lift part of the end cam 25. In a rolled seam, the lower pawl 24 is engaged with a lower lift part of the end cam 25 by a rotational operation of the dial 12, and the upper pawl 23 is detached from the tension disc 20. Thus, tension which is provided by the auxiliary thread tensioner (15) in addition to the main thread tensioner (4) is applied to the lower looper thread (9), and it is not necessary to carry out tension adjustment in switching between the over-edge chain stitch and the rolled seam.