38 and the upper plate 36 and between the separator plate 38 and the spacer plate 16 respectively the control goes to the step hs6 only when it is found that both upper and lower work fabrics are present at the respective positions in the step hs6 the contact pressure control routine shown in fig9 is executed so that the stepping motors 134 and 138 are operated according to the data stored in the optimum contact pressure registers reg20 and reg30 whereby the upper and lower rotary wheels 130 and 132 are brought into contact with the upper and lower work fabrics with the optimum contact pressure successively a step hs7 is performed to permit execution of the interruption routine in the subsequent steps up to hs12 therefore priority is given to the interruption routine which is repeated every about one millisecond to accomplish a cyclic control of guiding the work fabrics so that the side edges are located at the predetermined reference positions in the next step hs8 the start signal sb is fed from the output port 230 to the sewing motor drive circuit 40 to turn on the sewing motor 242 therefore a sewing cycle is conducted to form successive stitches along a line inwardly spaced a fixed distance from the side edges of the work fabrics while the side edges are guided so as to be located at the reference positions then the control goes to a step hs9 to check if both detection signals sd5 and sd6 representing the trailing edges of the work fabrics are 34 0 34 when both signals sd5 and sd6 are 34 0 34 a step hs11 is performed after a lapse of time in a timer routine of.