To provide an EC diaphragm unit which is suitable for the diaphragm unit, etc., of a photographing device such as a camera, specially, a camera with a lens, easily made small-sized, and has greatly improved response speed and repetition durability, and a high-performance film unit with a lens using the same.

The EC diaphragm unit has an electrolyte layer held between a couple of transparent electrode, each of the electrodes having a semiconductor nano-porous layer formed on at least one of its surfaces, arranged so that their semiconductor nano-porous layers are opposite to each other and can freely vary in a diaphragm diameter by reversibly changing from a transparent state to an opaque state. The semiconductor nano-porous layer is formed in multi- layered structure and the electrolyte contains at least one kind of EC dye which reversibly colors or decolors through at least one of electrochemical oxidation and reduction, and the film unit with lens which uses the same.