To be able to cope with emergency situations of a gas supply apparatus and with applications in which consumption amounts vary, and be able to minimize the energy consumption of a gas supply system as a whole with only a low loss of a product gas and the energy, even in such situations.

A reference gas amount Go to be fed to a consumption facility is set; a gas to be delivered from a gas supply apparatus 1 and a gas to be released to a vent flow path L4 are respectively controlled in pressure and flow amount so that the reference gas amount Go is secured by a gas amount Gs to be fed from the gas supply apparatus 1 to a diverting point M; and a differential amount dG in the case of a desired gas amount G to be fed to the consumption facility exceeding the reference gas amount, and a desired gas amount G when the gas supply apparatus 1 stops or a rapidly decreasing state of the supply amount is generated are controlled so as to be secured by a gas amount Gw to be fed from a backup apparatus 2 to the diverting point M.