To obtain a heat medium boiler that is capable of reliably reducing NOx concentration independent of the combustion amount, by mixing fuel gas and exhaust gas sucked into a fuel supply passage and mixed at an always constant ratio.

This heat medium boiler is provided with a recuperator 9 which preheats the combustion air by performing heat exchange between combustion air sent to a burner 1 using a fan 7 and the exhaust gas discharged from the boiler in which gas fuel is combusted. In a fuel gas supply line 10 are provided a flow regulating valve 11 which regulates the flow rate of the fuel gas in order to obtain a prescribed air ratio in accordance with the amount of air supplied to the burner 1, and an ejector 12 in which the fuel gas is the drive fluid on the secondary side of the flow regulating valve 11. The exhaust gas is sucked in by the ejector 12, is mixed with the fuel gas and combusted.