An overlock sewing machine includes an upper knife which is movable to a withdrawn position or a driving position, an upper knife driving unit which moves the upper knife, an interlocking switching unit which switches between an interlocking state in which the upper knife driving unit and the upper knife are interlock and a release state in which the upper knife driving unit and the upper knife are released, an upper knife position switching unit which switches the upper knife to the driving position or the withdrawn position, and an operating unit which drives the interlocking switching unit and the upper knife position switching unit such that the interlocking switching unit is brought into the interlocking state when the upper knife is switched to the driving position, and the interlocking switching unit is brought into the release state when the upper knife is switched to the withdrawn position.