A wideband magnetotelluric survey was performed in the Sengan geothermal region of northeastern Japan. A conductor with resistivity of < 30 Ωm was found at a depth of -1.8 km in the Quaternary Kakkonda granite. Microseismic activity was not observed, suggesting a ductile zone with temperatures exceeding 370 °C. Under these conditions, H2O-NaCl fluids can exist as two-phase or single-phase types. The permeability of the upper reservoir surface is between 5.0E-17 and 5.0E-15 m2, indicative of an exploitable supercritical geothermal reservoir. Our results indicate that this is originated by heat and water supply system from deep magmatic fluids.