NOVELTY - A bonding method involves forming a bonding film on a surface of a base member by plasma polymerization, where the bonding film contains a silicon skeleton of a random atomic structure containing a siloxane bond and leaving groups bound to the silicon skeleton. UV light is applied to the bonding film to eliminate the leaving groups at a surface of the bonding film from the silicon skeleton so as to provide adhesion properties to the bonding film. An accumulated amount of light transport vehicle (LTV) light is adjusted to control a refractive index of the bonding film.    USE - Method for bonding two base members to each other via a bonding film to produce a bonded structure that is utilized for manufacturing an optical element (all claimed) such as optical lens, diffraction grating, optical filter and protection plate, photoelectric conversion element such as solar cell, optical storage medium such as optical disk, and display element such as LCD element, organic electroluminescent element and electrophoretic display element.    ADVANTAGE - The method enables strongly bonding the two base members to each other via bonding film, thus forming the bonded structure with high size precision. The bonding structure has excellent thermal and chemical resistance properties, bonding strength and mechanical strength, and high light induced damage resistance.    DETAILED DESCRIPTION - A bonding method involves forming a bonding film on a surface of a base member by plasma polymerization, where the bonding film contains a silicon skeleton of a random atomic structure containing a siloxane (Si-O) bond and leaving groups bound to the silicon skeleton. UV light is applied to the bonding film to eliminate the leaving groups at a surface of the bonding film from the silicon skeleton so as to provide adhesion properties to the bonding film. An accumulated amount of the light transport vehicle (LTV) light is adjusted to control a refractive index of the bonding film. The base member and an object are bonded together via the bonding film to obtain a bonded structure.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of a wavelength plate.    Wavelength plate (9)    Birefringent crystal plates (91, 92)