Abstract: A hemming guide control for an overlock sewing machine which assures stitching of portions of the fabric to be sewn just forward of and behind a cross seam. The apparatus includes a hemming guide unit having a fabric guide surface, which is movable laterally with respect to the direction of sewing, structure for detecting the cross seam longitudinally behind the fabric guide surface, and structure, responsive to the detection of the cross seam, for moving the hemming guide unit away from the fabric to be sewn just before the stepped cross seam crosses beneath the sewing needle, and returning the hemming guide unit to its original position after the cross seam passes under the sewing needle, thereby to eliminate excessive pressure on the cross seam by the fabric guide surface along a segment of the sewing path from just forward to just rearward of the cross seam.