A frame member, which is approximately U-shaped in side view, has an ejector having elastic locking pieces whose tips are configured to move along long holes formed in an upper plate, mounted thereon. A permanent magnet is mounted on a slider member, at the end portion closer to a Hall IC switch. Pairs of elastic engagement pieces are inserted through the long holes respectively, and engaged with engagement portions provided at the tips of elastic engagement pieces of the ejector respectively and thus assembled. In a base member formed approximately rectangular-frame-shaped in plan view, the Hall IC switch is fitted on a Hall IC switch supporting extension, extended approximately perpendicularly outward from an end portion closer to the frame member of an outer plate positioned adjacent to the permanent magnet.