An attenuated total reflection (ATR) method for surface plasmon polariton (SPP) excitation was studied to improve the sensitivity level of the classical Raman scattering from adsorbates on a metal surface. The enhancement of the Raman band intensity using both in- and out-coupling of the SPP is 400 for copper phthalocyanine (CuPc, thickness t = 2.0 nm) on silver. The resonance condition and the enhancement factor substantially depend on the thickness and dielectric properties of adsorbates, gap media, and metal as theoretically predicted : (1) increased resonance angle of 0.3 degrees (in air) or 1.0 degrees (in water) by CuPc adsorption (t = 6 nm); (2) the optimum thickness of adsorbate, ca. 2.0 nm, for the largest enhancement; (3) the additional enhancement (ca. 4-fold) in water compared to in air gap media; (4) the optimum wavelength, e.g. 514.5 nm for a CuPc/silver sample, due to dielectric dispersion of the constituent.