A first housing (10) and a second housing (30) are connected by leverage brought about by the engagement of pinions (24) of a lever (20) rotatably supported on the first housing (10) and racks 33 of the second housing (30). In the process of rotating the lever (20), cam action brought about by the engagement of cam grooves (25) of the lever (20) and cam followers (37) of the second housing (30) is exhibited. This cam action prevents the first housing (10) from being inclined with respect to the second housing (30) with engaged positions of the pinions (24) and the racks (33) as supports.