PURPOSE: To aim at the promotion of small capacity in a spare gas storage system by separating fuel gas other than the specific load from a generator when city gas supply is stopped and pressure to an engine becomes less than the specified value, while supplying the engine with the fuel gas from a spare gas supply system.

CONSTITUTION: A gas system transfer controller 22 closes a valve 6 and opens a valve 15 instead when city gas supply out of a feed pipe 7 is stopped and pressure P1 in a gas pipe 4 being detected by a pressure gauge 20 goes down to less than the specified value, and feeds an engine of a gas generator 1 with spare gas out of a spare gas storage tank 19 via a valve 18 and a pressure reducing valve 17, controlling the engine 2 by an engine controller 8 according to output of a generator 3. Simultaneously switches 11bW11n of power distribution systems 12bW12n excepting 12a are turned off via a switch control panel 13. And, when pressure P2 to be detected by a pressure gauge 21 goes down to less than the specified value, the engine 2 is stopped while a switch 11a is turned off. Thus, the promotion of small capacity in a spare gas storage system is contrivable.