Method of heating a side-burner type heating furnace for slab

A method of heating slabs by the use of a side-burner type heating furnace having flame length-variable type burners on both side walls thereof, each burner consisting of an inner air flow nozzle, a fuel gas nozzle and an outer air flow nozzle arranged concentrically, the method comprising arranging the fuel gas nozzle of each burner between the inner air flow nozzle and the outer air flow nozzle, adjusting the ratio of the flow rate of air passing through the inner air flow nozzle to the flow rate of air passing through the outer air flow nozzle depending upon the variation of the flow rate of fuel gas, and further carrying out a side-by-side alternate burning or a zigzag alternate burning at a constant cycle when the turn down ratio is low.