PURPOSE: To prevent adhesion of acid ammonium sulfate and facilitate removing of dust by a method wherein steam is injected from the communication holes of a hollow tubular soot blower having injection nozzles arranged radially in a hollow rotary driving shaft through the distribution nozzles.

CONSTITUTION: A plurality of medium temperature soot blowers 15 are attached to a driving shaft 18, communicated with the communication holes 19 and a plurality of injection nozzles 20 are arranged. A steam pipe 16 is contacted with the communication hole 19 of the driving shaft through the distribution nozzle 17 arranged at the extremity end thereof, the distribution nozzle 17 is lightly pushed against it by a spring, the steam pipe 16 is fixed and communicated with the communication hole in the driving shaft 18 as well as with that of the nozzle 17 at a position of the blower 15 so as to inject the steam. Therefore, the blower 15 performs a dust removing action is sequence in response to rotation of the air preheater, so that even if the acid ammonium sulfate is apt to be adhered at a position of the medium temperature element 11, the temperature is increased with the blower 15 with the result that the acid ammonium sulfate in the element 11 is peeled off and even if it is adhered to the pipe, it may easily be removed.