We report the structures and the electronic properties of a novel terphenoquinone derivative having a benzo[1,2-b:4,5-b'] dithiophene (BDT) unit as a central ring. This molecule exhibits the open-shell biradical characters resulting from the thermally accessible open-shell structure. X-ray structure analysis and magnetic measurements clarify that this molecule has a closed-shell ground state, which is lower than the open-shell biradical state by 10.4 kJ mol(-1). The electronic absorption and electrochemical measurements reveal the small HOMO-LUMO gap about 1.3 eV of this molecule.