The estimation of underground water saturation is essential in geothermal fields, particularly for an enhanced geothermal system (EGS). Recently, electromagnetic exploration using magnetotellurics (MT) has been applied to the geothermal fields for estimating water saturation. However, the relationship between the electrical impedance obtained through this method and the water saturation in the reservoir rock has not been well known. Our goal is to elucidate this basic relationship via laboratory experiments. In the fluid-flow test, at first, reservoir rock samples were filled with nitrogen gas, which emulates the superheated steam observed in the geothermal fields. Then, brine was injected into the samples and its injection pressure was increased and decreased to vary the water saturation in the samples. During the test, water saturation, permeability, electrical impedance (at a frequency of 10m-100K Hz) and elastic wave velocity were measured. As a result of fluid-flow test on andesite (Makizono lava formation, Japan), the electrical impedance dramatically decreased from 100K to 1K ＆#8486; because of the brine injection. This remarkable change could be due to the replacement of pre-filled nitrogen gas with the brine. After the brine injection, the electrical impedance decreased with increasing injection pressure (small changes in water saturation) by up to 40%. In the pressure-decreasing phase, the electrical impedance increased with decreasing injection pressure and this electrical impedance was smaller than that observed in the pressure-increasing phase (up to 27%). However, the P-wave velocity was almost constant (less than 1%) at that time. These results indicate that the electrical impedance varied with small changes in water saturation in the pressure-decreasing phase, whereas P-wave velocity did not show any variations. In other words, this suggests that electrical impedance could be sensitive to minor changes in water saturation compared with P-wave velocity.