To automatically control a damper opening in order to set a gas volume by employing a gas volume achievement data of a damper opening, arranging a control table of the damper opening and gas speed, and carrying out two steps control for finely regulating the gas volume so as to set an achievement value to a command value on the basis of the table.

In a material carry line, in the case where a gas volume is controlled for forcibly controlling iron having a high temperature, gas speed is detected by an anemometer 11 arranged on an outside of a damper 6, its detecting signal is inputted to a control operating panel 12 through a converter 9, and a gas volume achievement value is found out. The achievement value is controlled by a host control calculator 13, and also a gas volume command is imparted on a rolling automatic setting board 14. The setting board 14 is provided with a control table for restricting a characteristic of the gas volume and a damper opening value, and the damper opening value is found out from a commanded gas volume. The achievement value after several seconds is detected. When its achievement value is other than a setting range, a command for finely operating the damper opening speed with super lowest speed is outputted.