Abstract: A boiler according to the present invention is a boiler configured to make combustion gas originating from combustion in a burner (101) flow from a furnace (102) and pass through a super heater (SH) (104) and an evaporation tube bank (105). The boiler includes a downstream shield plate (11A) that is slidable in the vertical direction of the super heater (104) at a position downstream of the combustion gas flowing above the super heater (104), thereby regulating the flow rate of combustion gas entering the upper space A of the super heater (104). By regulating the flow rate of bypass gas (12) with the downstream shield plate (11A) and in turn regulating the flow rate of mainstream gas (13), the temperature of steam output from the super heater (104) is controlled.