PURPOSE: To provide a method of reducing concentration of water in a superheater in order to prevent scale, etc., from being produced in the superheater when a boiler is stopped.

CONSTITUTION: In a flue and smoke tube boiler not requiring blow-off, for preventing concentration of water in a water chamber part in a flue from being increased, the water in the water chamber part is fed to a superheater to form steam, and the steam is used such that the water in the water chamber part is prevented from being increased in its concentration with the same principle as continuous blow-off. A blow-off apparatus 8 is provided which is constructed such that when the boiler is stopped, water in the superheater 4 concentration of which is increased is discharged to the outside.