A thread cutter for a sewing machine includes a first thread seizing assembly reciprocally movable and having a distal end with a first thread seizing portion, a second thread seizing assembly seizing needle and bobbin threads both having been seized by the first assembly during a backward movement of reciprocation of the first assembly, cutting the needle and bobbin threads in cooperation with a cutting blade, and a thread cutting frame supporting the first assembly and formed with a single elongated groove supporting the first assembly so that the first assembly is movable. The elongated groove includes a linear proximal end groove, an oblique portion, and a main groove and being shaped such that the proximal end groove is translated slightly forward through the oblique portion. When moved backward in the reciprocation, the first assembly is swung so that the first thread seizing portion comes close to the second assembly.