There is disclosed an overlock sewing machine. The overlock sewing machine includes a looper switching unit for selecting an overlocking operative state or overlocking inoperative state, a needle clamp member having a first needle holding hole in which two needles are mounted separately in the manner that the two needles are positioned in parallel and a second needle holding hole positioned slightly eccentrically at the front side of the cloth feeding direction and perpendicular to the cloth feeding direction, whereby eight types of overlocking operations and one type of double-loop stitching operation can be effected.