PURPOSE: To eliminate the production of a defective article and to improve the yield of products by accurately adjusting a preheating zone in a firing furnace to a desired temp.

CONSTITUTION: A preheating zone 6 and a firing zone 7 are arranged in parallel close to each other in a U-shaped hair pin conveying line extending from the inlet to outlet of an enamel firing furnace, a heater 11 is provided on both sides of the preheating zone 6 just as in the firing zone 7, and a temp. sensor 13 is furnished between the heater 11 and an article 9 to be fired. The heater 11 is controlled by a temp. controller (not shown in the figure) based on the signal from the sensor 13, and the zone 6 is kept at a desired temp. Consequently, the progress of firing is appropriately controlled, and a defective article is not produced.