A combustion air supply apparatus 9 of alternating heat exchange type supplies combustion air and discharges combustion exhaust gas at a flow velocity of 80 to 200 m/sec. A burner assembly 4 is configured in such a manner that low-caloric fuel gas is pre-heated with heat of pre-combusting high-caloric fuel gas before the low-caloric fuel gas reaches a mixing starting space CA in the combustion chamber where the pre-combusting high-caloric fuel gas and the low-caloric fuel gas come to burn together in a full scale in the mixing starting space CA. When an air amount of the combustion air supplied through the high-temperature air supply ports of the plurality of fuel gas combustion apparatuses is defined as Q1 and an air amount of the pre-combustion air to be mixed with the high-caloric fuel gas, supplied from the fuel gas combustion apparatuses, is defined as Q2, a total air amount (Q1+Q2) is 1.02 to 1.10 times more than a theoretical air amount QS required for combustion, and a ratio of Q2/(Q1+Q2) is 0.011 to 0.047.