The automatic adsorption apparatus has been developed to measure the individual adsorption isotherms of constituent vapor from binary mixed vapor. The present adsorption apparatus consists of a combination of gravimetry and volumetry. The weight increase due to gas adsorption was determined by the magnetic suspension vacuum balance with a sensitivity of 0.01 mg, while the pressure decrease due to adsorption was measured by a high precision pressure gauge with a sensitivity of 0.2 Pa. The adsorption data obtained from both gravimetry and volumetry make it possible to determine the individual isotherms of the constituent vapors without chemical analysis, if the molecular weights of the constituent vapors are significantly different one another. In the later part of this work, the present adsorption apparatus has been applied to measure the individual adsorption isotherms of ethanol and water by active carbon fiber from their mixed vapor. The reliability of the individual isotherms measured by the present apparatus has been considered. The adsorption data obtained in the present work clearly indicate that ethanol molecules are preferentially adsorbed into the micropores of active carbon fiber with a pore width of 0.67 nm. The present adsorption result suggests the possibility of the energy-saving separation of ethanol from the mixed vapors of ethanol and water.