<p>An object is to obtain a desired dryness and reduce the risk of corrosion of a boiler body while suppressing the height of an upper header. A boiler is provided with: a boiler body (20) in which the water level inside water pipes (23) when combustion by a burner (30) is stopped is lower than the upper ends of the water pipes (23); water supply means (70); control means (100) which controls the operation of the water supply means (70) depending on the water level in the boiler body (20); header water level detection means (60) which detects a water level existing inside an upper header (24); and a downcast pipe (84). The control means (100) performs first control for controlling the operation of the water supply means (70) so as to lower a boiler body water level when the water level detected by the header water level detection means (60) reaches a header set water level and second control for controlling the operation of the water supply means (70) so as to raise the boiler body water level when the amount of drop in the boiler body water level reaches a predetermined amount after the first control.</p>
