The electronic structure of an ionized high spin molecule, m-phenylenebis(phenyl-methylene), is studied with a semi-empirical model for π and n electrons using the unrestricted Hartree-Fock (UHF) and resonating HF (Res HF) approximations. The injected hole or electron is in the π system and the calculated spin multiplicity of the ground state is the quartet, which is consistent with the experiments. In the UHF approximation, it makes a spin density wave (SDW) polaron localized around a carbene site. The Res HF approximation shows that quantum motion of the SDW polaron between the carbene sites and quantum fluctuations of its spins more stabilize the quartet state.