of a line line of action along which the pressing forces of the press maneuver parts 24 25 are directed accordingly the stoppers 72 74 are subjected to the pressing forces of the press maneuver parts 24 25 at the proximity of the line of action therefore in a case where the engagement levers 44 46 are rotated in a direction for disengaging releasing engagement the stoppers 72 74 restrict the rotation of the engagement levers 44 46 by making contact against the arcuate groove portions 76 78 respectively in such a case the engagement levers 44 46 are positively restricted from rotating without being bent since the stoppers 72 74 are situated at the proximity of the line along which the pressing forces upon the press maneuver parts 24 25 are directed the torsion springs 48 50 are attached to the engagement levers 44 46 in a manner so that coil portions 48 a 50 a are housed in the groove portions 56 58 respectively of the engagement levers 44 46 in addition since the torsion springs 48 50 are coiled around the outer periphery of the shafts 64 66 the torsion springs 48 50 are attached to the engagement levers 44 46 in a compact.