We proposed a new measuring method of the binding constant K-B, by proving that the inclusion degree a in a host-guest reaction is equal to the rise ratio of rejection eta in ultrafiltration. The K-B for nitrophenol (NP) and cyclodextrin (CD) system was measured. The data were compared with those of other investigators. The K-B measured by the proposed method decreased with increase in operating pressure, but a constant value of K-B was obtained below a certain pressure. The pressure was nearly 1 MPa for the NP-CD system. The constant value was the equilibrium binding constant K-B. With an increase in pH, the K-B became large because guest molecules were charged and the anion and hydrogen bonds between host and guest molecules became strong. With a decrease in the concentration ratio epsilon of guest molecules to host molecules, the inclusion degree alpha became large, but the binding constant K-B remained almost constant. After checking the validity of the new method by NP-CD system, the K-B value for optical isomers of amino acids, threonine and phenylalanine, were measured to investigate whether the optically selective inclusion of guest molecules by CD was possible or not. CD, which is an optically active D-form molecule, tended to include optical L-acid rather than D-acid under the present conditions. The optimum conditions were referred to resolute optical isomers.