PURPOSE: To improve thermal efficiency by sucking part of low temp. gas from an exhaust port or near the same, boosting the gas with boosting fans and injecting the boosted low-temp. waste gas into high-temp. waste gas flow in positions where materials to be rolled are preheated with the high-temp. waste gas in the preheating furnace.

CONSTITUTION: Part of waste gas having lowered to low temp. is sucked from the exhaust port 5' or the exhaust end of a preheating furnace, and is boosted with boosting fans 6, 6'. This boosted low-temp. waste gas is passed through distribution pipings 7, 7', and is ejected as jet flow into high-temp. waste gas flow by injection nozzles 8, 8' in the positions where materials 1, 1' to be rolled are preheated in the high-temp. waste gas in the preheating furnace. The jet flow collides against the surfaces of the materials 1, 1' by carrying the high-temp. waste gas, and flows along said surfaces.