The invention relates to an electrochromic (EC) device for controlling transmittance of light, such as a window pane or a filter for a display. The EC device has a double-decker structure produced by superposition of two identical EC cells each having two oppositely arranged EC electrodes one of which is formed of a first EC material that takes on color in oxidized state, such as Prussian blue, and the other of a second EC material that takes on color in reduced state such as WO.sub.3. The EC device has a transparent inner substrate with an EC electrode layer using one of the first and second EC materials on each side thereof and two transparent outer substrates each with an EC electrode layer using the other of the first and second EC materials. There is a peripheral seal between the inner substrate and each outer substrate, and the space defined between the inner substrate and each outer substrate is filled up with an electrolyte liquid. To minimize the number of leads for application of voltages to the four EC electrode layers, the EC electrode layers on the both sides of the inner substrate are connected with each other by an angled strip of a sheet metal fitted on the inner substrate, and the EC electrode layers on the outer substrates are connected with each other by another angled strip of a sheet metal fitted into a marginal region of the space between the two outer substrates.