Continuous measurements of vertical subsurface geoelectric fields in the frequency range dc – 0.6 Hz and 1 – 3 kHz (VLF) are reported. Anomalous field variations of the ULF band are observed a few days prior to, and a month after, a minor volcanic eruption of Mt. Mihara. The variations displayed essentially distorted pulses, rising sharply initially, gradually decreasing, and then suddenly falling off. The electric field variations also seem to extend to the VLF band. It is felt that the anomalous electric field variations were caused by electrokinetic phenomena induced by variations of hydrothermal circulation around the crater, or by variations of crack density, due to volcanic activ