PURPOSE: To prevent plastic image formation elements from the reduction of its optical performance due to deformation, deterioration, etc. based upon the humidity absorption of the elements by coating the whole surface of the plastic image formation elements with a heat contracting high molecular film, heating and contracting the film and then removing the contracting film at the transmission part of image forming light flux.

CONSTITUTION: When a roofmirror lens array 6 is to be constituted by combining and storing the image formation elements consisting of a plastic lens array 2, a plastic roofmirror array 3, an optical path converting member 4, etc. in a housing 5 for instance, the whole roofmirror array 3 is coated with the heat contracting high molecular film 11a, heated and adhered to a contracting film 11b, and then the contracting film 11b on the reflecting surface 3a of the roofmirror array 3 is removed to form an opening part 11c. Thus, the array 3 is prevented from humidity absorption by a humidity absorbing layer 11 of the contracting film to prevent the array 3 from deformation, size change, deterioration, and the reduction of optical performance.