Abstract: A low-NOx boiler comprises a burner for burning gaseous fuel, heat transfer pipes heating water by combustion heat of the burner to generate steam and having a passage through which water flows, a steam drum communicating with the heat transfer pipe and accumulating the steam from the heat transfer pipe, and an exhaust gas duct located in the furnace to exhaust combustion gas generated by combustion of gaseous fuel outside of the boiler. The boiler further comprises unburnt gas burner in the exhaust gas duct for burning unburnt content in the combustion gas flowing in the duct. The unburnt gas burner includes a nozzle for injecting premixture flow of fuel and combustion air and flame holding plate located near the downstream side of the outlet of the nozzle to interrupt the premixture flow to form a circulating flow on downstream side of the flame holding plate. As gaseous fuel and air are mixed in advance and a flame is formed, the flame length can be shortened.