PURPOSE: To optimize the concentration of oxygen for combustion air to be supplied to a main body of a boiler by a method wherein an oxygen enriching device is arranged in the midway part of a combustion secondary air duct downstream of a branch point of a double stage combustion secondary air duct.

CONSTITUTION: An oxygen enriching device 13 is arranged in the midway of a combustion secondary air duct 10 downstream from a branch point of a double stage combustion secondary air duct 11. With such an arrangement as mentioned above, when a part of the secondary air passes through the oxygen enriching device 13, nitrogen is removed to increase the oxygen concentration. The air passes through an inner vane 4a of a burner 2 and an outer vane 5a of the burner 2 and is supplied from an inner annular supplying passage 4 and an outer annular supplying passage 5 into the main body 1 of boiler. As a result, the small-sized oxygen enriching device can favorably be applied and a large- sized forcing blower 8 can be obviated, resulting in that the oxygen concentration of the combustion secondary air supplied to the main body of the boiler can be optimized.