A functional film, which has photo-catalytic effects, is overlaid on a surface
of each of at least two substrate materials. The functional film, which has been
overlaid on the surface of each of the substrate materials, is exposed to light
having a wavelength falling within an absorption wavelength range of the functional
film. The substrate materials are then bonded to each other with the functional
films, which have been exposed to the light, intervening between the substrate
materials. The functional film may be a film containing TiO2, and the
light irradiated to the functional film may be ultraviolet light. In cases where
a film other than the functional film is also overlaid on the surface of each of
the substrate materials, the functional film is formed as a top layer.
