To provide a boiler capable of realizing a reduction in O2, a reduction in NOx, and a reduction in CO. The present invention provides a boiler including: a premixed gas burner, and water tubes in close proximity to the premixed gas burner, characterized in that the premixed gas burner ejects a premixed gas toward the water tubes at a predetermined angle; and the boiler further includes a fuel supply portion capable of supplying at least one of a gas fuel and a premixed gas provided at a position on a downstream side of and spaced apart by a predetermined distance from the premixed gas burner.