The present invention has as its object to provide a thin type optical system which enables both of image information displayed on display means such as a liquid crystal display to be observed and image information of which image is substantially equal to the image observed via therethrough and which is taken by image pickup means and a compound display apparatus using such optical system. The modification includes a wheelchair lift adjacent the rear of the vehicle. The retaining forces between the mechanical closure means and the outer face of the main part are lower than the retaining forces between the mechanical closure means and the outer face of the side parts in the front area. The system page reduces execution overhead without sacrificing system security. A liquid return passage, in communication with the return channel, is positioned through the second wall and overlaps the outlet opening of the liquid dispensing channel as viewed from a direction perpendicular to a surface of the first wall of the liquid dispensing channel.