With a overlock sewing machine, when a switching mechanism is switched to a threading-enabled state, and a looper cover is closed, the looper cover is slid toward one side in the hinge shaft axial direction so as to operate a detection lever. In the non-operating state, the detection lever is set to an initial position where it does not press an operation protrusion of a switch. When a side cover is closed, and the detection lever is operated, the operation lever is set to an operation-enabled position where it presses the operation protrusion. When the operation lever is operated, and the side cover is opened, the detection lever is set by passing through the operation-enabled to the operation-disabled position where it does not press the operation protrusion.

A throat plate switching mechanism includes a first throat plate and a second throat plate. The first throat plate is formed with a first needle hole having an elongated hole shape at a stitch point. The second throat plate is formed with a second needle hole. A width of the second needle hole is narrower than a width of the first needle hole. The second throat plate is equipped in the first throat plate such that the second throat plate is switched into a use position at which the second needle hole overlaps the first needle hole, and a retreated position at which the second throat plate does not cover the first needle hole.

A throat plate switching mechanism includes a first throat plate and a second throat plate. The first throat plate is formed with a first needle hole having an elongated hole shape at a stitch point. The second throat plate is formed with a second needle hole. A width of the second needle hole is narrower than a width of the first needle hole. The second throat plate is equipped in the first throat plate such that the second throat plate is switched into a use position at which the second needle hole overlaps the first needle hole, and a retreated position at which the second throat plate does not cover the first needle hole.