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.