PURPOSE: To control the pressure of a gaseous atmosphere with a simple system, by purging automatically the gas from an annealing furnace with a solenoid valve which is closed after a prescribed time by a timer prior to annealing, detecting the pressure in an inner cover during annealing and regulating the opening of a diaphragm valve.

CONSTITUTION: A gaseous atmosphere is supplied with a valve 9 through a flowmeter 6 and a solenoid valve 14 from a part 11 into a furnace according to the kind and application of a steel strip. Since the valve 14 is beforehand determined of the time for opening and closing by a timer, the valve opens for a prescribed time and is then closed. The burner 5 of an outer cover 1 is ignited upon ending of the purging and the supply of the atmosphere into the furnace in this case is accomplished through a diaphragm valve 13 which is opened by the pressure at a pressure detecting end 10. The valve 13 is beforehand regulated to maintain the pressure in the furnace at, for example, 50mm, and even if the valve 13 is closed when the prescribed pressure is attained, about 1m3/hr gaseous atmosphere is supplied.