PROBLEM TO BE SOLVED: To provide a medium having reversible variable permeability against an electromagnetic beam by thinning a solution layer showing high stability against circulation of an electric potential difference between electrodes and sufficiently lowering concentration of an anode compound and a cathode compound in a solution without causing precipitation and so as to reduce a segregation problem. SOLUTION: A spacer or a segregating layer 11 holds flat electrode holding plates or walls 110 and 130 with a thin interval between them and in parallel on a device to assemble and surrounds a space or volume 12. A solution to fill the volume 12 contains a solvent and at least one kind of anode compound and at least one kind of cathode compound and is low in concentration, and it makes contact with both of electrodes 10A and 13A during actuation of the device. Sheet resistance of an electrode layer is to be 1-40 ohm per square. Consequently, it is possible to reflect light intensity allowable to a driver regardless of intensity of light causing glare incident upon a mirror from a headlamp of an automobile approaching from the rear.