A theory that gives the existence domain of the complex dielectric constant of the binary mixture for given volume fractions of the components is presented as an extension of O. Wiener's theory. This domain is represented in the complex plane by the overlapped region of two circles. The present theory has the advantage of obtaining the result without any information on the state of mixing, although it does not give a definite value. The correspondence between the point in the domain and the state of mixing is studied. The examples of application of the theory and some notes are shown.