Geophysical prospecting methods have been used for explorations of oil, mineral and geothermal energy resources, and recently are applied to civil engineering and environmental problems. We carried out magnetotelluric surveys, which is a kind of electromagnetic explorations, to investigate the electrical resistivity structure of the seismogenic region of the 1962 Northern Miyagi Earthquake (M.6.5) in Miyagi Prefecture, northeastern Japan. As a result, a deep conductor was found and is interpreted as a fluid-filled zone. We suggest that the seepage of the fluid from the fluid-filled zone to the resistive granitoid pulton can become a trigger of the earthquakes.