To provide a combustion device and its method for estimating the property of a second fuel by simultaneously burning a first fuel and the second fuel having different property from the first fuel.

The technology for estimating the property of the second fuel is provided on the basis of the property of the first fuel, operating information of a combustion furnace 310, flow rates of the first fuel and the second fuel by a property calculating device 370. The technology is provided for mixing the tag identifying the kind of fuel, to the fuel, conveying the fuel together with the tag, reading out the tag mixed in the conveyed fuel, and monitoring the supply of the fuel containing the tag on the basis of the tag reading time and the tag information. The technology for simultaneously burning the first fuel and the second fuel having the property different from the first fuel in the combustion furnace 310 on the basis of these technologies, is provided.