The purpose of the present invention is to decrease the loss of energy of flue gas when superheated steam is cooled and prevent heat efficiency from being decreased. A boiler is provided with: economizers (a medium-pressure economizer (13) and a high-pressure secondary economizer (18)) which heat water supplied by water supply pumps (a medium-pressure water supply pump (27) and a high-pressure water supply pump (28)); evaporators (a medium-pressure evaporator (16) and a high-pressure evaporator (21)) which evaporate the water that is heated by the economizers; and cooling devices (a medium-pressure system and a high-pressure system) which mix water, as a coolant, which has passed through the economizers via the water supply pumps with steam.