PURPOSE: To prevent an upward camber at free ends not supported of horn members of a cantilever by uniformizing an upward pressure loaded to an engaging surface of jaw members between from a front end to a back end of upper jaw members of the cantilever.

CONSTITUTION: In order to dissolve an upward camber at end parts 47, 48 not supported of horn members 41, 42 of a cantilever which is generated by an upward pressure generating at the time of tightening operation, a transfer structure consisting of (a) a vertically transferable members 39, 40 and (b) a transferable lever member 59 which ascends and descends the sliding members is used. Upper jaw members 58, 54 of a tightening member are attached to the bottom part of the horn members 41, 42 of the cantilever. An upward pressure loaded to an engaging surface of the jaw members 58, 54 between a front end to a back end of an upper jaw parts 53, 54 of the cantilever is uniformized by the transfer structure. Thus, the upward cumber at free ends 47, 48 not supported of the horn members 41, 42 of the cantilever is dissolved.