A twin-needle sewing machine for stitching lap seams which have been produced by basting a pair of fastener stringers to the edges of an article bounding an opening therein and then by folding the article edges along the basting lines. The sewing machine has a presser foot comprising a pair of parallel spaced main portions, and a pair of wings projecting laterally outwardly from the respective main portions. A guide fixedly mounted on the throat plate of the sewing machine has its sloping rear surface disposed opposite to the front end of the presser foot. The folded article edges together with the stringers basted thereto follow a curved path created by the guide as they are drawn under the presser foot for final stitching of the lap seams, in order that the article edges may not unfold.