In an overlock sewing machine, a main shaft fixing operating shaft is provided with a differentiation ring that can be swung in a predetermined range such that, when the rotation of a main shaft is to be enabled, it is set to an enable state in a stationary state, and when the rotation of the main shaft is disabled, it is swung and shifted from the enable position. Furthermore, a looper cover open/closed detection configuration is configured by modifying a part of a typical known configuration. Such an arrangement provides an overlock sewing machine having a simple configuration that requires only a single micro switch to detect the switching between the threading mode and the sewing mode and to detect the open/closed state of the looper cover.