Response spectra of strong-motion accelerograms obtained in Japan were dealt with statistically using a multiple regression technique and the spectra at a rock site were scaled in terms of earthquake magnitude. Papageorgiou and Aki's model was considerd as an useful inhomogenous faulting model to explain the spectra. After incorporating the theoretical spectra due to the model with the statistical ones, a scaling law for the paramerers of inhomogeneous fault was derived. It was shown that the localized crack radius of inhomogeneous fault increases with earthquake magnitude M, giving a size of about 1km for M=6 and about 11km for M=8, and the total number of the cracks on the fault is camparatively stable irrespective of M, namely, it ranges from 10 to 20 at most between M=6 and M=8.