PURPOSE: To reduce calorific values of each burner, to decease radiation of a flame and to uniform the temperature distribution in a furnace by dividing the burners into groups, arranging the groups at both sides of the furnace, and adding temperature regulating means, etc., to the respective groups.

CONSTITUTION: 2-12 of burners 1 each having a calorific value of 1000-5000kcal/ hr are disposed at an interval of 50-250mm on a sidewall of an upper part and/or a lower part of a roller conveyor 3 at both sides of a furnace to constitute a burner subgroup 2, and the subgroups are arranged at a pitch of 200-2000mm. Burners groups 5, 5' of one system are formed of 1-5 of burner subgroups 2 at one side and burner subgroups 2 of the same number opposed thereto. Further, temperature detecting means 20 and temperature regulating means 19 are added to the respective groups 5, 5'. Thus, the calorific value of one burner 1 is reduced to decrease radiation of a flame, and the temperature distribution in the furnace is made uniform.