To provide a gas storage adjusting method for a gas holder capable of securely preventing an overflow of by-product gas stored in the gas holder even if the by-product gas storage rapidly increases.

In the middle of a by-product gas supply pipe, a gas holder for storing a surplus of the by-product gas is disposed for the purpose of adjusting the excess or deficiency of the by-product gas. The gas holder is characterized in that a pressure of the supply pipe is controlled according to a pressure range from a gas pressure attributable to a piston weight inside the gas holder at the control start value of the gas storage of the gas holder to a gas pressure inside the gas holder when the piston descends at the upper limit value of the gas storage of the gas holder set for the use limit of the gas holder.