PURPOSE: To prevent lowering of practicality of a furnace due to upward extensive dispersion of a burner and to prevent contact of a furnace wall with flame by disposing burners of different types of fuel at four corners and sides of a horizontal square section.

CONSTITUTION: One flame retardant fuel burner 4 and an assist burner 8 are provided at corners, and other flame retardant fuel burner 6 is disposed at a center of the side of a furnace. The burners are so disposed as to regulate angles in a horizontal direction. The burners 4 are disposed in multi-stages at the corner, the burners 8 may be disposed therebetween, and the burner 5 may be disposed in multi-stages at the side. Thus, in the case of an existing facility, a decrease in an allowable load is suppressed, lacking in a furnace is prevented, and a heat load at the side of the furnace in which the heat load is concentrated is alleviated.