Geomagnetic depth soundings have been made in the area covering Kyushu island and its neighboring islands in the western sea of Kyushu to investigate the back-arc conductivity structure. The in-phase induction arrows derived from geomagnetic variations are found to point roughly southwestwards at most sites on Kyushu island for the period range 300--7200 sec. This is quite different from some other areas of Japan. The non-uniform thin sheet model calculation indicates that the induction arrows in this area are strongly influenced by the surrounding seas. However, the observed arrows have much more intense westward components than indicated by the model calculations. This strongly suggests existence of highly conducting layers (HCLs) beneath the East China Sea. And the existence of HCLs in the upper mantle are confirmed by a two-dimensional finite element modeling. These HCLs may be considered to be a part of mantle upwelling extended to East China Sea.¿ 1997 American Geophysical Union