A structure is realized that is capable of preventing interference of respective friction plates (39, 40a, 40b) with other members existing therearound or with the body of a driver, and furthermore that facilitates, when assembling, insertion of a tension rod (17a) through a circular hole (47) and longitudinal elongated holes (18a, 18b) formed in the respective friction plates (39, 40a, 40b). The respective friction plates (39, 40a, 40b) are arranged between both of the side faces of a displacement bracket section (36) and the inner face of supporting plate sections (22a) that constitute a fixed side bracket (15a). Moreover, the end edges of the respective friction plates (39, 40a, 40b) symmetrically present in pairs are connected by connecting sections (45, 46a, 46b), to thereby integrate the left and right friction plates (39, 40a, 40b) so that the respective holes (45, 18a, 18b), through which the tension rod (17a) is to be inserted, can be easily aligned.