PURPOSE: To reduce the passing speed in an annealing furnace without changing the characteristics of an Al sheet material after an annealing by simultaneously changing the annealing temp. of the sheet material and an air nozzle pressure for heating during the treating time with devices before and after the annealing furnace.

CONSTITUTION: The band-shaped Al sheet material supplied from an un-coiling device 4 is passed via a sheet joining machine 5 and an inlet side accumulator 2 into the floating type continuous annealing furnace 1 consisting of a heating furnace 11 and a cooling furnace 12 where the sheet material is continuously annealed. The sheet material is then taken out via an outlet side accumulator 3 and is taken up on a take-up device 7 via a sheet cutting machine 6. The annealing temp. of the above-mentioned sheet material and the air nozzle pressure for heating are simultaneously changed during the treating time of sheet joining, sheet exchanging, sheet cutting, etc., with the devices before and after the annealing furnace 1 to reduce the passing speed in the annealing furnace 1 without changing the sheet material characteristics after the annealing. The treatable time with the devices before and behind the furnace is prolonged in this way without changing the sheet material characteristics, by which the accumulators 2, 3 are shortened and the prevention of the the trouble with the annealing furnace 1, etc., are enabled.