A thin plate-like switch unit comprises a board having a plurality of contacts, and a sheet-like rubber contact member superposed on the board, the rubber contact member having convex switch portions for opening and closing the plurality of contacts, respectively. A side cover covers the switch unit fixedly mounted on a side surface of a seat portion. Windows are formed through the side cover, and the switch portions are exposed through these windows. Flanges are formed at peripheral edge portions of the windows, and projects outwardly beyond the switch portions. Preferably, the switch portions are arranged in corresponding relation to directions of movement of the seat.