PURPOSE: To perform parallel operation of a gas holder as respective gas holders are controlled to a given level without increasing an equipment cost even when a communication piping between the gas holders is long.

CONSTITUTION: A pressure regulating valve 9 is arranged in a piping 13, being shorter, of pipings 12 and 13 running from blast furnaces 3 and 4 to gas holders 1 and 2. A control means 8 sets the pressure value on the upper stream side of the pressure regulating valve 9 based on a difference in a level between the two gas holders 1 and 2, and a pressure regulating valve 9 is controlled so that a generating gas pressure detected by a gas pressure gauge 7 is adjusted to a value responding to a set pressure value. In which case, a feed net work is separated into a high pressure line situated upper stream from the pressure regulating valve 9 and a low pressure line situated downstream therefrom by the pressure regulating valve 9, and a gas flow rate flowing to the two gas holders 1 and 2 is increased or decreased through the increase or the decrease of a set pressure value on the high pressure line side. This constitution performs absorption and smoothing of a generating gas pressure fluctuation by the pressure regulating valve 9 simultaneously with control of the levels of the two gas holders 1 and 2 to a constant value.