A gas having high CO content evolved at reaction zone in a ferrosilicon-refining electric furnace is burned and deflected to wipe transversely on the top level of the charged raw materials with the aid of air introduced near electrodes through an air pipe. The burning of the gas is so controlled that a content of oxygen in the burned gas is within a range of 12-17% by volume by controlling a volume of the air to be introduced near electrodes as well as a volume of air invading into the furnace through a poking window and a gap between a shell of the furnace and a heat-shielding hood above the furnace. The CO-rich gas which has heretofore been exhausted in vain from the furnace can now be used to preheat the charged raw materials and prevents the furnace from blocking due to formation of crusts at the top level of the charged raw materials. The exhaust gas is now less noxious, has a higher temperature that can be used for driving a turbine of a generator. High heat efficiency, high quality of ferrosilicon, useful electric power and less blocking troubles can be obtained in a simple and economical way.