To prevent alkali corrosion of a superheater by providing a means for separating and evaporating in advance mist in saturated steam flowing into the superheater of a boiler.

Saturated steam from a steam drum 1 flows from a first saturated steam pipe 2 to a superheater inlet header 3, rises through a second saturated steam pipe 10, and branched into a horizontal pipe which is shorter than a horizontal pipe of a first superheater 4 of a longitudinal second superheater 11. While the saturated steam rises through the second saturated steam pipe 10, the mist in the saturated steam is settled and separated, and discharged as a drain from a drain pipe 12 and a steam trap 13 provided on the superheater inlet header 3. The mist which is not settled and separated by the second saturated steam pipe 10 is evaporated while flowing through the second superheater 11, making mist including NaOH difficult to flow to the first superheater 4. By reducing the mist flowing into the first superheater 4 in the above manner, alkali corrosion in the first superheater 4 is suppressed.