We have investigated the characteristics of S coda waves on a sedimentary basin for examining if they may be applicable to estimate the stable amplification factor. The results are as follows: (1) R. M. S. amplitude ratios of coda waves observed on the basin to coda waves on the nearest rock site were inconsistednt with the ratios of S waves between the above two sites for frequecies lower than 3Hz. The ratios of coda waves scattered as much as those of S waves. (2) The phase velocities determined for coda waves on the basin were consistent with those of Rayleigh waves estimated by another study. (3) Comparing coda waves observed on the basin with those synthesized through convolving the SH wave response for the basin and the seismogram on the rock site, the former was much smaller than the latter. From the above three results, coda waves on the basin seem to consist of not only single-scattering S waves but also surface waves generated locally. It is therefore difficult to estimate reliable amplification factor from coda waves observed on the sedimentary basin.