A compound of the following formula: Ea—La—X—Lb—Eb in which each of Ea and Eb independently is a group having oxidation-reduction activity and having a conjugated system in its group; X is a divalent cyclic group; and each of La and Lb independently is a group which does not form a conjugated system in combination with the conjugated system of each of Ea and Eb and at least one of which has a site imparting water solubility to the compound or a site that is convertible into a site imparting water solubility to the compound, is favorably employable as an electroconductive threading intercalator in an electrochemical method for detecting complementary DNA fragments.