A salt-forming compound represented by the following general formula (3):




where X1 and X2 each independently represent an aromatic ring group optionally containing a substituent; Y represents a divalent hydrocarbon group that the carbon atom directly bound to X1 or X2 does not have a π bond; Z+ represents an organic cation group; e represents an integer of from 1 to 4; and when e is 2 or more, a plurality of Ys and a plurality of Z+ s may be each the same or different; Ac- represents a heteropolyoxometalate anion which is a c-valent anion and which has an oxidation-reduction potential larger than −0.3 V relative to the silver/silver chloride electrode; f and c are each an integer of 2 or more; g is an integer of 1 or more; and the salt-forming compound is a normal salt that f×e=c×g.