We propose a new equation of the absolute adsorption isotherm for supercritical gas and examine the effectiveness of this equation using density functional theory (DFT). The new equation can well describe both DFT results and experimental data of a high-pressure adsorption isotherm of a supercritical gas. This equation provides important parameters of the averaged fluid-pore wall and fluid-fluid interaction energies, suggesting that adsorbed layer has two states. One is an attractive state in low-pressure region and another is a repulsive interacting state in high-pressure region. In the case of DFT results of nitrogen adsorption on the regular slit pore of pore width = 2 sigma (sigma: Lennard-Jones size parameter of an adsorptive molecule), the obtained enthalpy change of this transition from Clausius-Clapeyron equation is 4.4 kJ/mol. It is slightly smaller than that of vaporization of bulk liquid nitrogen at boiling point (5.57 kJ/mol).