To avoid inconvenience caused by the environment for setting a storage tank and the weather by a dome part covering a floating roof, to prevent a fire occurring on the floating roof by a ventilation shutting means, and to swiftly and smoothly extinguish the fire by means of the smothering fire extinguishing.

The dome structure 10 is mounted on a storage tank 1 comprising a storage tub 6 for storing a flammable liquid and the floating roof 7 mounted movably inside the storage tub 6. The dome structure 10 comprises the dome part 11 covering the floating roof 7, supports 12 disposed inside the storage tub 6 for supporting the dome part 11, a vent 14 disposed on a leakage preventive panel 13 supported by the supports 12 for making the inside and outside of the dome part communicate with each other, and the ventilation shutting device 15 for shutting the vent 14 when a fire occurs on the floating roof 7.