PURPOSE: To reduce the generation of NOx a great deal by providing a burner head which has a plurality of primary flame openings and a primary combustion chamber which has a secondary flame opening and forming a pre-mixing chamber in the primary combustion chamber.

CONSTITUTION: The area surrounding a burner head 11 which has a Plurality of primary flame openings 12 is covered by a primary combustion chamber 13, and the upper section of the primary combustion chamber 13 is provided with a secondary flame opening 14. A pre-mixing chamber 15 is also provided in the upper section of the primary flame opening 12. Because of divided the combustion into two stages, primary and secondary, the combustion load is reduced and the temperature of combustion flame becomes low. Further, at this time the pre-mixing gas in the pre-mixing chamber 15 and the combustion gas in the primary combustion chamber 13 exchange heat and as a result the temperature in the pre-mxing gas rises very much and the combustion stabilizes. On the other hand because the temperature of the environment in the primary combustion chamber 13, the temperature in the combustion flame 18 and the temperature of the portion that is left unburned in the primary combustion are kept from rising, the combustion speed in the secondary combustion drops, and the temperature rise in the secondary combustion flame 19 are suppressed, reducing the amount of NOx formed in the primary and secondary combustions a great deal.