To reduce heat loss during the supply of a molten material molten by the heat of arc by forming the peripheral edge of the opening part of an arc furnace in the shape of a circular arc about the central axis of tilting of a furnace caging, separating the peripheral edge of the opening part from a storage tank, and tilting the furnace casing while the storage tank is stationary.

While a storage tank 23 above an arc furnace is located as it is (stationary state), a furnace casing 1 is tilted leftward. The tilting angle is, for example, about 15 to 25°. This tilting movement causes a molten steel in the furnace casing 1 to be supplied from the steel supply port 12 of a steel supply trough 11. During the tilting movement, the peripheral edge 17 of the opening part 16 of the furnace casing 1 does not interfere with the storage tank 23, since the peripheral edge 17 is formed in the shape of a circular arc. Since the substantial part of the opening part 16 of the tilted furnace casing 1 is kept covered with the opening part 30 of the storage tank 23 and a part of the opening part 16 which is not covered with the opening part 30 is also covered with a covering member 32, the discharge of heat from the opening part 16 is lowered. Thus, heat loss during the supply of steel can be reduced.