A back mirror has plural mirror surface areas of different curvatures disposed side by side or plural mirror surface areas disposed side by side to form discontinuous planes. The back mirror includes a dividing line having a light diffusion effect formed along a border portion of the plural mirror surface areas. The dividing line may be formed by projections and depressions having a light diffusion effect which are formed on at least one of a transparent substrate, reflecting film and a transparent thin film formed on the reflecting film.