A monoanion of biphenyl-3,3’-diylbis(phenylmethylene) (BP-3,3’-BPM) with a low-lying quintet and a triplet state near the singlet ground state was generated by gamma-radiolysis followed by a subsequent visible light photolysis in a frozen solution of 2-methyltetrahydrofuran. BP-3,3’-BPM was chosen as a prototypical example whose electronic spin structure is characterized by the possession of an intramolecular weak exchange interaction between two carbenic units. A spin-quartet state was detected for the monoanion of BP-3,3’-BPM at 77 K by ESR spectroscopy. The ground state was determined to be a spin doublet from the measurement of the temperature dependence of the ESR signal intensity of the quartet state. The energy gap between the doublet ground and the quartet excited states was estimated to be about 35 cm(-1). The effective exchange interaction between the two carbenic units of the : monoanion was compared with that of the neutral BP-3,3’-BPM to conclude that the pi-spin polarization dominates in the spin alignment of the monoanion.