To provide a new method for removing a carbonaceous scale in which the carbonaceous scale deposited in a furnace and containing at least carbon is removed from the furnace.

The method for removing a carbonaceous scale comprises introducing carbon dioxide gas into a reactor 10 as a furnace through a carbon dioxide gas-introducing pipe 40, reacting the carbon dioxide with a carbonaceous scale 20 to convert the carbonaceous scale 20 to carbon monoxide gas, and exhausting the carbon monoxide gas from the reactor 10 through a second exhaust pipe 30.