The purpose of the present invention is to maintain the intake pressure of a water supply pump at an operable pressure. A boiler is provided with: condensate pumps (a condensate pump and an auxiliary condensate pump); a branch line that causes water delivered by the condensate pumps to branch; a drum (a low-pressure drum) that is connected to one (a low-pressure branch line) of two lines into which the branch line branches; and a water supply pump that is connected to the other (a high-pressure branch line) of the two lines into which the branching line branches and that pumps water to an evaporator (a high-pressure evaporator). The boiler is additionally provided with pressure applying means that guides a portion of the water in the drum to the water supply pump side when the intake pressure on the inlet side of the water supply pump has become lower than a predetermined pressure.