The interfacial electronic structure of a CO-covered polycrystalline platinum electrode has been studied by using the optical second harmonic generation (SHG) and sum frequency generation (SFG) techniques with various excitation wavelengths. Although the nonlinear optical (NLO) signal was enhanced by the absorption of CO at all excitation wavelengths employed in this study, the potential dependent behaviors of the NLO signal were different between the near-infrared excitation and the visible excitation. The difference was attributed to the different mechanisms for the enhancement of the NLO signal. While the electron transition from the Fermi level of Pt to the unoccupied 2 pi(a)* orbital of adsorbed CO was considered to be coupled with the NLO photon in the case of visible excitation, the dc field induced SHG regime was applied to the result obtained by the near-infrared excitation.