Abstract: A chaining thread sew-in device of an overlock sewing machine is related having a first pinching plate and a second pinching plate which form a part of the working face of the sewing machine on the operator side of a needle drop point. The plates are capable of opening an insertion hole into which a free end side of a chaining thread linked to the sewing machine side can be inserted in the working face by driving the first pinching plate, capable of pinching the chaining thread between itself and the second pinching plate, toward the second pinching plate. By returning the first pinching plate to the initial position while the chaining thread is inserted into the insertion hole a cutter 30 capable of crossing the chaining thread cuts the chaining thread by moving the second pinching plate 71 to the first pinching plate 71 side in when the chaining thread is inserted into the insertion hole.