An overlocking sewing machine capable of easily performing rolled sewing without the need of any troublesome tension adjusting operation. This machine is provided with an auxiliary thread tensioner (15) separately from near a main-thread tensioner (4), by which yarn tension required for forming edge linking is applied to a lower looper thread (9). An end cam (25) is provided on a shaft (13) of a dial (12) for feed adjustment such that when a lower pawl (24) engages with a high lift portion of the end cam (25), an upper pawl (23) pushes in one location on a peripheral edge of tension dishes (20) to expand a side of the tension dishes opposite to the location so that the lower looper thread (9) passing between the tension dishes is released from application of tension and is subjected to application of tension only by the main-thread tensioner (4) for normal edge linking. When rolled sewing is performed, turning operation of the dial (12) causes the lower pawl (24) to engage with a low lift portion of the end cam (25) to disengage the upper pawl (23) from the tension dishes (20). Accordingly, tension is applied on the lower looper thread (9) by the auxiliary thread tensioner (15) as well as the main-thread tensioner (4), so that it is unnecessary to perform adjustment of tension at the time of switching between edge linking and rolled sewing.