PURPOSE: To reduce fuel unit and shorten the furnace length of a heating furnace by providing a preheating furnace in connection to the heating furnace and conveying sheet steels to the heating furnace after preheating these with the exhaust gas of the heating furnace.

CONSTITUTION: The sheet steel 9 conveyed by charging side table rollers 4 is first carried into a preheating furnace 21 in proximity to or continuation to a heating furnace 12. Therein, it is preheated by the hot wind from jet nozzles 24, 25 of the exhaust gas of the heating furnace 12 sent through a duct 13 by a fan 14. Thence, it is carried into the heating furnace 12, where it is uniformly heated to predetermined temp. by radiant tubes 7, 8.