Overlock sewing machine

An overlock sewing machine, comprising:

• an upper knife movable to a retreat position below a needle plate or a projection position above the needle plate, and, when moving vertically from above the needle plate, capable of cutting a sewing object in cooperation with a lower knife;
• an upper knife drive part for vertically moving the upper knife;
• an interlock switching part configured to switch an interlocked state where the upper knife drive part and the upper knife are interlocked and a released state where the upper knife drive part and the upper knife are not interlocked each other;
• an upper knife position switching part configured to switch the upper knife between the projection position and the retreat position;
• and,
• an operation part configured to drive the interlock switching part and the upper knife position switching part to switch the upper knife position switching part between the following three modes, that is, a first mode where the interlock switching part provides the interlocked state and the upper knife position switching part sets the upper knife at the projection position, a second mode where the interlock switching part provides the released state and the upper knife position switching part sets the upper knife at the projection position, and a third mode where the interlock switching part provides the released state, the upper knife position switching part sets the upper knife at the retreat position.