PURPOSE: To provide the increased capacity and pressure for storage by compressing and storing gaseous by-product and to provide systematization by combining compressors and a gas turbine generator with gas holders.

CONSTITUTION: Gaseous by-product at a low pressure is compressed with compressors 2, 5 to a middle or high pressure. The compressed gas is stored in gas holders 3, 6, and the pressure in the gas holders is divided to multiple stages according to need so that the gas is conducted successively from the holder 3 for middle pressure to the holder 6 for high pressure. Piping systems 7, 9 leading to utilizing installations are provided to the gas holders in the respective stages, and when the evacuation is required, the evacuation is accomplished while energy is recovered with a gas turbine 8. As a result, the more gaseous by-product can be stored in a limited space and the release of energy resources is considerably suppressed. The electric power required for the compression stage for compressing the gas to middle and high pressure is utilized in the night time when the price is low, and since the energy required for the compression is recoverable by the power generation with the gas turbine in the stage of the evacuation, the economical effect in terms of electric consumption is high.