To provide a duct damper capable of completely stopping leakage of gas, by cutting off a flow of the gas in a duct.

This duct damper 10 includes a valve seat part 20 having an opening 21 for making the gas flow, a valve element part 30 for opening-closing the opening 21, and a driving part 40 for driving the valve element part 30. The valve seat part 20 has a water tank 22 formed by surrounding the periphery of the opening 21. The valve element part 30 has a seal plate 32 inserted over the whole periphery in the water tank 22. The water tank 22 has a water supply port, a discharge port and an overflow port.