There is disclosed a secondary battery wherein either one of positive electrode and negative electrode elements is made of an active material the color of which is changed according to a voltage between these electrodes and a window is formed for observing the color change of the electrode. Further, an electrochromic display device is disclosed in which a working electrode is made of a pyrolytic carbonaceous material which is obtained by thermal decomposition of a hydrocarbon compound at a low temperature of about 1,000 DEG C.
