Electric power application for electro-osmotic dewatering has been generally investigated under conditions of either constant voltage or constant current. If the electrical resistance between two electrodes holding a material such as sludge increases with dewatering, the dewatering process has to be stopped when the voltage required to maintain a constant current is too high, while the process operated at constant voltage stops because the current passing through the material gradually decreases. The increase or decrease of the electrical resistance between the electrodes depends on the physical and electrical properties of the material to be dewatered. In view of these properties, a combination of constant voltage and constant current was investigated as a means to improve the dewatering process. The combined operation was shown to be effective in terms of the amount of water removed and the efficiency of electric power consumption for its removal.