The effect (Stark effect) of an applied electric field on the electronic states of molecular adsorbates was studied by measuring surface plasmon resonance (SPR) as a function of the wavelength of the incident light that excites the SPR. Using the Kramers-Kronig relation, Stark spectra comparable to those obtained with conventional methods were extracted from the electric field-induced SPR angular shift for several organic adsorbates. Because this method relies on detecting the SPR angular shift that can be measured precisely, high-sensitivity Stark spectroscopy can be achieved. In addition, the adsorbate coverage information can be determined from the SPR angular shift upon molecular adsorption.