We propose a simple method for identifying interaction strength between an adsorbate molecule and a solid surface. The interaction strength is necessary for precise evaluation of pore sizes of the nanometer order, in which the conventional Kelvin model fails to describe the condensation phenomena, and for which we proposed an alternative model taking account of the effect of attractive potential from pore walls on the condensation: The model contains a constant to express the interaction strength between an adsorbate and a solid surface, and it should be determined from a standard isotherm. For investigation and verification of the method, standard adsorption isotherms were prepared by the grand canonical Monte Carlo simulations for the system of a Lennard-Jones particle on a homogeneous solid surface. The Frenkel theory was basically applied for this purpose because of its explicit inclusion of the interaction parameter within the equation. The theory shows good agreement with simulation results in a limited range of relative pressures from 0.005 to 0.25, while certain deviations exist in other portions of the isotherm. The result is that only the portion of the standard isotherm with good agreement should be used to evaluate the interaction strength employing the Frenkel theory. Through examinations in systems with various interaction strengths, it is found that the interaction strength is able to be evaluated correctly if the equation is applied to the range of surface coverage from 0.8 to 1.8 in all cases. The interaction constant calculated by this method is proved to have sufficient accuracy for use in characterization of nano-scale pores.