A method of forming a strong seam by using a sewing machine, without backstitching. In the sewing machine, a first shuttle hook which rotates in the counterclockwise direction, a first needle, a second needle, and a second shuttle hook which rotates in the counterclockwise direction are arranged in turn in the feed direction. A material to be sewn is fed every time when the needles simultaneously and independently form stitches, so as to form two independent seams which overlap each other in a manner such that the knots of the seams overlap each other on each knotted point, where one of the seams consists of perfect stitches, and the other seam consists of hitch stitches.