To provide a method of stably controlling the switching of fuel in a boiler, and a boiler device, in switching the supply of a part of fuel to a furnace of the boiler at work with a main fuel from the main fuel to a gas fuel.

The plurality of burners have burner sections of prescribed stages to a boiler unit applying pulverized coal fuel as the main fuel, and the gas fuel accompanied by the generation of BOG (boil off gas) of LNG (liquefied natural gas) excluding the main fuel can be supplied to the burner of the first stage. Control is performed so that the influence of the supply of gas fuel is gradually increased according to the decrease of the supply of the main fuel at a P1 time point in the burner of the first stage, and the supply of the gas fuel reaches a prescribed amount at a switching point α taking air at the neighborhood of a main fuel stop time point P7 when the main fuel is not supplied. The ignition timing (P3 time point) of the prescribed burner in the burner of the first stage is shifted, and the switching point α is moved at the neighborhood P7 of the main fuel stop time point.