The present invention detects, using a single switch (84), the opening/closing of a looper cover (110) and a side cover (130), and the switching of a threading state. This overlock sewing machine (1) is configured so that when a switching mechanism is switched to a sewing state, the looper cover (110) held in a closed position is slid to one side in the axial direction of a hinge shaft (94), thereby actuating a detection lever (76). In a non-actuated state, the detection lever (76) is held in an initial position in which the detection lever (76) does not press an actuation protrusion (84A) of the switch (84). When the detection lever (76) is actuated while the side cover (130) is in a closed position, the detection lever (76) is held in an actuation permitting position in which the detection lever (76) presses the actuation protrusion (84A). When the detection lever (76) is actuated while the side cover (130) is in an open position, the detection lever (76) is moved through the actuation permitting position and held in an actuation prevention position in which the detection lever (76) does not press the actuation protrusion (84).
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