The electronic structure of Pb(II) porphyrin (PbP) has been studied by performing ab initio calculations based on a density functional theory. The calculated optimum structure of PbP reasonably agrees with that determined by the X-ray structure analysis. The analysis of the Kohn-Sham orbitals of PbP has revealed that the bond between Pb and the porphyrin ring (Pb-P bond) is mainly constructed from 6p(z) and 7s orbitals of Pb and 2p(x) and 2p(y) orbitals of the four nitrogen atoms. It has also been demonstrated that a Lewis base, CH3-, adds to Pb by attacking the unoccupied Pb-P antibonding orbital, which is a counterpart of the Pb-P bonding orbital mentioned above. This mechanism can be also applied to another Group 14 metalloporphyrin and Lewis base bond.