The Tohoku-Shinkansen line was constructed more than 30 years ago, and the electrical equipment was replaced in the last decade. At the time, however, the grounding systems of the substation were not replaced, because it was assumed that the grounding system had not deteriorated. In this study, we confirm the influence of stray current on the grounding system of the Tohoku-Shinkansen line in certrain ways. The philosophy of a grounding system for the electric railway of Japan is different from that of the international standard. The influence of stray current caused by the D.C. railway on the grounding system of the traction power supply system is confirmed by 30 years' experience of the Tohoku-Shinkansen Line and evaluated quantitatively based on the measured data. Such results are valuable to prove the reliability of Japanese style grounding system and will be instructive to other Asian countries where railways are emerging in the future.