stitches when the dial 13 is adjusted to an even number i e when the last bit of the code of the dial value in fig6 is 0 such as 00110 the code corresponds to the position coordinate 10010 of the pulse motor in this case the basic zigzag stitches are fed by a constant feed amount x 3 subsequently if the feed adjusting dial 13 is rotated to slightly increase the feed amount and set to the value corresponding to 00111 there is no corresponding position coordinate of the pulse motor however in this case the micro computer of this invention produces a discriminating signal n which alternates between 0 and 1 at each stitch if n 1 at the first stitch the micro computer is so operated to calculate that the adjusted position or value of the dial 13 is 00111 1 and to produce a position coordinate 10010 which would ordinarily correspond to the dial value 00110 and the pulse motor 2 is driven to produce feed amount x 3 when n 0 at the subsequent stitch 00111 1 is calculated to produce a coordinate 01000 and the pulse motor 2 is driven to provide feed amount x 4 as a result zigzag.