Mesozoic crystalline mountains (Iide Mountains) in a nonvolcanic region of the Northeast Japan Arc were imaged by wide-band magnetotelluric soundings. A two-dimensional model shows that an anomalous conductive body (<10 Ωm) is clearly visible beneath the Iide Mountains. The conductor widens with increasing depth and extends from the near-surface down to the base of the crust and perhaps into the upper mantle. The location of the conductive body correlates with high-temperature hot springs with high 3He/4He ratio, thinning of the brittle seismogenic layer, and anomalies of low seismic velocity. We conclude that the conductor reflects the presence of partial melts in the crust, related to renewed magmatism in the present-day subduction system.