To produce a lens having an antireflection function in a satisfactory yield. A dielectric multilayer film (3) is formed on one surface (12a) of a lens main body (12). An aluminum-containing film (14a) is formed on the other surface (12b) of the lens main body (12). The aluminum-containing film (14a) is immersed in warm water to thereby change the aluminum-containing film (14a) into a film having a finely rugged structure and comprising an alumina hydrate as a main component. Thus, a lens is produced in which both surfaces (12a, 12b) have an antireflection function.