PURPOSE: To obtain a light quantity control diaphragm device which does not cause a drop of the light quantity, and consists of a simple structure, by providing a light shielding member which contains a material whose light transmittivity is changed electrochemically by applying a voltage and crosses an optical path.

CONSTITUTION: A light shielding member is constituted of each layer of a glass substrate 2, the first transparent electrode 3, a first coloring layer 4, an insulating layer 5, a second coloring layer 6, and the second transparent electrode 7. Both the first coloring layer 4 and the second coloring layer 6 consist of an electrochromic (EC) material which can be oxidized and reduced electrochemically, cause oxidation - reduction reactions by an applied electric energy, and varied to that which has a transition probability by which an electronic state of a substance is different from zero against a light beam of a visible area. That is to say, a light quantity control diaphragm control can be executed electrically, therefore, the operation is executed easily, the structure is simplified, and also, since the EC material based on an electrochemical reaction is used, the light quantity scarcely drops.