An improved overlock machine is proposed which is easier to set a thread through a looper. The members which would obstruct the access to the looper are mounted on a movable portion (45) of a bed frame. By swinging open the movable portion (45), the obstructing members are moved to their inoperative position. A fixed knife (5) only, or both the fixed knife (5) and the movable knife (11) are mounted on the movable portion.