PURPOSE: To supply the atmospheric gas in a low cost and to stably produce a high quality magnetic steel sheet by executing refining and component-adjustment to the used atmospheric gas in a continuous annealing decarburizing furnace and circulating to reuse the atmospheric gas.

CONSTITUTION: At the time of executing a primary recrystallize annealing containing the decarburization after cold-rolling, the used atmospheric gas from the part at the inlet side of the steel sheet 2 in the continuous annealing furnace 1 is taken out through an atmospheric gas exhaust pipe 4 and an atmospheric gas circulating device 10 and sent to an atmospheric gas refining device 8 and refined. In the refining device 8, the used atmospheric gas is changed to a regenerated atmospheric gas composed of H2, inert gas and <1% innevitable impurities of CO2. Further, in an atmospheric gas component adjusting device 9, this regulated gas is made to be ≥25% H2 concn., <1% CO2 and the balance the inert gas and 40-70°C dew point by adding H2-containing gas having H2 conc. lower than that in the furnace 1 atmosphere, the inert gas for adjusting component and H2O, and supplied to the outlet side part of the steel sheet 2 in the furnace 11 through a supplying pipe 3 fox atmospheric gas and fed to the reverse running direction of the steel sheet 2.