PURPOSE: To decrease fuel consumption in a soaking furnace for indirect heating of an Al casting ingot provided with a damper by controlling the opening and closing of the damper under a specific condition thereby regulating the flow rate of fuel.

CONSTITUTION: An Al casting ingot contained in a furnace is heated at the fuel flow maintained at a specified rate with respect to said ingot while the quantity of the hot wind to be circulated in the furnace is limited. The limitation for the quantity of the hot wind is released when the furnace temp. attains 300W400°C, then the flow rate of the fuel is increased to maintain the monotonous increase of the furnace temp. thereby maintaining the fuel flow at a specified rate. The fuel flow is returned to the original specified rate and the ingot is heated in the stage of having no need for limiting again the quantity of the hot wind with an increase in the furnace temp. The fuel is again increased and the ingot is heated at the specified fuel flow rate just before the prescribed furnace temp., that is, at the point when the gradient of the heating curve decreases and the fuel is minimized on approaching of the heating capacity to a threshold.