PURPOSE: To form uniform oxide layer to a strip and to stabilize decarbonization by detecting concn. of detecting flow gas flowing plural zones, comparing with the permissible value and controlling opening degree of atmosphere partition according to this difference.
CONSTITUTION: He of the detecting flow gas is poured into heating zone 2 and soaking zone 3 in a continuous annealing furnace and dry gas is supplied into slow cooling zone 4 and also the atmosphere partition 6 is closed. Atmospheric gas in the furnace flowing from the soaking zone 3 to front part side of the heating zone 2 is flowed out from an atmospheric gas exhaust hole 8. The detecting gas concn. C 1 in this gas, is detected with a detecting gas concn. detector 12. The detecting flow gas concn. C 2 in the atmospheric gas flowing to the atmosphere partition 6 side in boundary with the slow cooling zone 4 is detected with a detecting flow gas detector 12a. Both detected values C 1 , C 2 and the detecting flow gas pouring quantity are inputted in an arithmetic device 13. By comparing different value between front flowing atmospheric flow rate and rear flowing atmospheric flow rate at the zones 2, 3 and the permissible value, the opening degree of atmosphere partition 6 is controlled.