  The out-of-plane dielectric constant of the parent insulator of the
high-temperature superconductor Bi_2Sr_2(Dy,Er)Cu_2O_8 was measured and
analysed from 80 to 300 K in the frequency range of 10^6-10^9 Hz. All the
samples were found to show a fairly large value of 10-60, implying some kind of
charge inhomogeneity in the CuO_2 plane. Considering that the superconducting
sample Bi_2Sr_2(Ca,Pr)Cu_2O_8 also shows a similar dielectric constant, the
charge inhomogeneity plays an important role in the insulator-superconductor
transition.
