PURPOSE: To prevent the suction of bubbles by feeding the low-temperature water from a water feed pump to a high-temperature water circulating pump when the after-cleaning mode is set in a system utilizing the energy of the hot water discharged from a gas engine to generate steam.

CONSTITUTION: The high-temperature water circulating pipe 13 of a steam tank 12 is connected to the suction port of a high-temperature water circulating pump 10, and the discharge port 14 of the circulating pump 10 is connected to an engine 16. The engine 16 is connected to the steam tank 12 via a hot-water pressure adjusting valve 18, and the steam of the high-temperature water stored in the steam tank 12 is fed to the user side via a steam pressure adjusting valve 20. A normally-closed blow solenoid valve 21 is provided on a pipe 22 communicating the steam tank 12 to the outside, and the discharge port 31 of a water feed pump 30 is connected to the high-temperature water circulating pipe 13. When the after-cleaning mode is set, the water feed pump 30 is operated until the level in the steam tank 12 reaches the upper limit value after the engine 16 is stopped, and the blow solenoid valve 21 is opened.