at both sides and in its top and in its rear portion on the stepped portion 3 there is mounted a front edge guide plate mounting plate 5 in flush relation with the top surface of lower base plate 1 through screws 6 6 the screws 6 6 are in thread engagement with threaded holes 3a 3a in the stepped portion 3 through elongate holes 5a 5a in the front edge guide plate mounting plate 5 at both sides thereof at the rear portion of the front edge guide plate mounting plate 5 there are formed a suitable numbers of kerfs 7 and comb teeth 8 in transversely spaced apart relation to the direction of fabric movement x the intervals of the comb teeth 8 correspond to the intervals of sewing needles not shown at the stepped portion 4 there is mounted a rear edge guide plate mounting plate 9 through screws 10 10 in flush relation with the top surface the screws 10 10 are in thread engagement with threaded holes 4a 4a in the stepped portion 4 through elongate holes 9a 9a in the guide plate mounting plate 9 at both sides thereof at the front portion of the guide plate mounting plate 9 that is at the opposite side of the front edge guide plate mounting plate 5 there are formed suitable numbers of kerfs 11 and comb teeth 12 in transverse relation to the direction of fabric movement x and at wider intervals than those of the kerfs 7 in the guide plate mounting plate 5 a recessed portion 40.