To enhance the jointing strength at a jointed part and to improve reliability for a long period of time and environmental temperature resistant characteristics without executing wet-treating by irradiating a functional film provided on the surface of a base material and having photocatalytic effect with light and jointing the base materials to each other through the films.

Functional films 11, 21 such as TiO2 thin films having photocatalytic effect are provided on each surface 10a, 20a of respective two base plate materials 10, 20, such as wafers. These functional films 11, 21 are each formed on each surface 10a, 20a by sputtering, a sol-gel process, or the like, after subjecting the each surface 10a, 20a to mirror finishing, and when a film other than it is also formed, this functional film is formed as the uppermost layer. Then, the functional films 11, 12 are irradiated with light 30 having the wavelength in the absorption wavelength region, such as ultraviolet rays, in order to generate photocatalytic effect, thereby fixed organic materials, or the like, are decomposed and removed and a super-hydrophilic state is formed. Thereafter, the both base plate materials 10, 20 are stuck together through the functional films 11, 21 heated, pressurized and jointed. Thereby, a composite base plate material having the functional films as an anti-reflection coating film thin enough to be optically neglected is obtained.