Abstract: Increase or decrease in a quantity of alumina presenting in a bath or a metal in an aluminum electrolytic cell is detected and a quantity of electric power supplied to the cell is increased or decreased in accordance with the result of detection. The detection is made by calculating a difference between a quantity of alumina supplied to the cell during an interval between an instant at which a previous anode effect has occurred or was anticipated to occur and an instant at which a present anode effect occurs or is anticipated to occur, and the quantity of alumina consumed during the interval.