CO2 sequestration in a deep aquifer is considered one of the most effective method to solve Global warming. We must understand the behavior of injected CO2 to confirm long-term stability and economic efficiency of CO2 sequestration. We tried to monitor the behavior of gas, liquid and supercritical CO2 injected into water-saturated sandstone by measuring the resistivity of rock specimens. CO2 of gas, liquid, and supercritical phases was flushed through the sandstone specimens at three different flow rates. We could monitor the time-lapse behavior of CO2 in water-saturated sandstones. Also, we tried to convert the resistivity data to the CO2 saturation using Archie's equation. The saturations estimated from resistivity becomes nearly equal to the values calculated from actual outflow volumes. This result indicates the high reliability of electric exploration to monitor CO2 saturation.