PURPOSE: In indicator which uses, as color producer, electrochemically reversible, oxidation-reduction indicating material, bipyridyl complex of transition metals is used in auxilliary redox system, to effectuate good reversibility and long service life of indicator.

CONSTITUTION: An indicator contains a color producing liquid which is contacted with two or more of electrodes. The color producing liquid contains electrochemically reversible, oxidation-reduction coloring material, such as N,N'-di (p-cyanophenyl)-4,4'-bipyridylium dichloride, and auxilliary redox system, which comprises 2,2'-bipyridyl complex of transition metals such as 2,2'-bipyridil complex of iron, as indicated by the general formula, (wherein M is a transition metal such as Fe, Ru, Co, Mn, Cr and Ni, n is number of X corresponding to oxidation number of M and X is anion such as Cl, Br, ClO4 and BF4).