To provide a desired foaming ratio without being affected by smoke during a fire even when a foam generator is provided inside a building.

In a high expansion foam fire extinguishing system, a protective object 1 is provided with the foam generator 10 provided with a net for foaming on the exit of a foam generator body incorporating nozzles and a fire in the protective object 1 is extinguished by foam discharged from the foam generator 10. The protective object 1 is provided with a ceiling plate 5, a space part at the back of the ceiling is formed as an air duct 3, the foam generator 10 is provided so as to pass through the ceiling plate 5, and the net for foaming is made to face the inside 9 of a room.