The present invention provides an industrial method that allows to produce an indole derivative trimer with high purity in mass, as well as novel indole derivative trimers obtainable by the method, having high conductivity, high oxidation-reduction potential, high oxidation-reduction capacity, and the excellent cycle characteristics. The present invention relates to a method of producing an indole derivative trimer comprising the step of oxidizing an indole derivative in a reaction solution containing an organic solvent and to the novel trimers produced by the method.