To prevent the occurrence of a skip stitch and a bird's nest state caused by the condition where, at the time of starting sewing by a thread with a sewing machine, a needle thread hooked on a needle is not fixed.

The sewing machine is provided with a knot forming device 13. The knot forming device 13 includes a knot forming part 21 for forming a knot of a needle thread 10 and a cutting part 22 for cutting the needle thread 10. After the end of a series of sewing work, with the needle thread 10 stretched between a piece of cloth 12 and a needle 14, a knot of the needle thread 10 is formed by the knot forming part 21. Subsequently, the needle thread 10 is cut just under the knot 11 by the cutting part 22 to provide the knot at the sewing start side end part of the needle thread 10. In the case of again starting sewing with the sewing machine, the sewing start side end part of the needle thread 10 is stopped on the upper surface of the piece of cloth 12 by the knot 11. Accordingly, the end part of the needle thread 10 is kept from being drawn in under the piece of cloth 12 so that the occurrence of a skip stitch and a bird's nest state can be prevented.