having thereon a stepped portion 20c which is opposed to a bent lug 21a formed on an operating bar 21 the operating bar 21 is mounted for back and forth movement at the center of the base plate 11 and has a knob 21b attached thereto and exposed outside the base plate 11 for manual operation thereof upon rearward manual movement of the operating bar 21 the bent lugs 21a thereon are engaged with the stepped portions 20c of the latching levers 20 to pivot the levers 20 from their latching position to an unlatching position indicated in phantom in fig4 against the urging of the springs 23 to thus release the slide plates 12 to permit the needle bed end parts 1b and 1c to be pulled out from the center part 1a on the other hand upon inward push of each end part 1b 1c towards the center part 1a the marginal edge portion of the opening 12e of the slide plate 12 is engaged with the inclined camming edge 20b of the latching lever 20 to pivot the lever 20 towards its unlatching position against the urging of the spring 23 so that the slide plate 12 and hence the end part 1b lc are permitted to return to its innermost position latched by the latching lever 20 in order to eliminate the necessity of very accurate positioning of a latching lever 20 relative to a slide plate 12 particularly in the moving direction of the plate 12 the latching edge 20a of the lever.