To effectively and safely start combustion in a combustion chamber, by igniting a burner by starting the combustion of fuel in the chamber before forming water films on both upper and lower surfaces of a conical porous plate by start of sprinkling water from above a heat absorbing chamber and directly above a top of the plate.

A burner 4 installed in a combustion chamber 3 is ignited to start combustion of fuel in the chamber 3, water sprinkling is started from above a heat absorbing chamber 5 and directly above a top of a conical porous plate 6, and an operation is started. In this case, before the water sprinkling is started by a cooling water nozzle 10a of a cooling water tube 10 from above the chamber 5 to a water supply nozzle 8a of a water supply tube 8 and from above the top of the plate 6, the burner 4 is ignited in the chamber 3 to start combustion of fuel. Thus, high fuel concentration in the chamber 3 is prevented, riskiness of explosion at the time of igniting is eliminated, and combustion of the fuel can be effectively and safely started.