An overlock sewing machine comprising: a threading mechanism configured to perform threading of a looper; a switching mechanism configured to switch the threading mechanism between a threading state and a sewing-enabled state; a looper cover coupled with a hinge shaft having an axial direction extending in a width direction of the overlock sewing machine on a front side of the threading mechanism such that it can be opened and closed, and configured to cover at least a part of the looper at a closed position; a switch including an operation protrusion, and configured such that, when the operation protrusion is pressed, a motor configured to drive a main shaft is switched from a driving-disabled state to a driving-enabled state; and a detection lever configured to press the operation protrusion, and configured such that, when the switching mechanism is switched to the sewing-enabled state in a state in which the looper cover is positioned at a closed state, the looper cover is slid toward one side in an axial direction of the hinge shaft, to operate the detection lever, wherein, in a non-operation state, the detection lever is positioned at an initial position at which it does not press the operation protrusion; characterized by further comprising a side cover provided on one side of the hinge shaft with respect to the looper cover, and coupled with a sewing machine main body such that it can be opened and closed; wherein, when the detection lever is operated in a state in which the side cover is set to a closed position, the detection lever is set to an operation-enabled position at which the detection lever presses the operation protrusion, and wherein, when the detection lever is operated in a state in which the side cover is set to an open position, the detection lever is set by passing through the operation-enabled position to an operation-disabled position at which the detection lever does not press the operation protrusion. a threading mechanism configured to perform threading of a looper; a switching mechanism configured to switch the threading mechanism between a threading state and a sewing-enabled state; a looper cover coupled with a hinge shaft having an axial direction extending in a width direction of the overlock sewing machine on a front side of the threading mechanism such that it can be opened and closed, and configured to cover at least a part of the looper at a closed position; a switch including an operation protrusion, and configured such that, when the operation protrusion is pressed, a motor configured to drive a main shaft is switched from a driving-disabled state to a driving-enabled state; and a detection lever configured to press the operation protrusion, and configured such that, when the switching mechanism is switched to the sewing-enabled state in a state in which the looper cover is positioned at a closed state, the looper cover is slid toward one side in an axial direction of the hinge shaft, to operate the detection lever, wherein, in a non-operation state, the detection lever is positioned at an initial position at which it does not press the operation protrusion; characterized by further comprising a side cover provided on one side of the hinge shaft with respect to the looper cover, and coupled with a sewing machine main body such that it can be opened and closed; wherein, when the detection lever is operated in a state in which the side cover is set to a closed position, the detection lever is set to an operation-enabled position at which the detection lever presses the operation protrusion, and wherein, when the detection lever is operated in a state in which the side cover is set to an open position, the detection lever is set by passing through the operation-enabled position to an operation-disabled position at which the detection lever does not press the operation protrusion.