PURPOSE: To prevent dust from adhering to a group of heat exchanger tubes, by making exhaust gas passing through the group of the heat exchanger tubes into a turbulence state when heat possessed by exhaust gas is recovered.

CONSTITUTION: A velocity of flow of exhaust gas G passing through a superheater 3 is detected always through a current detector 8 by a control box 7 which stores and applies an instruction signal, and a damper 10 of an exhaust gas supply pipe 9 is turned open when the velocity of flow of the exhaust gas measured is lowered to a critical velocity of flow which has been applied beforehand. With this, a part of the exhaust gas whose dedusting is completed by a dust collector 6 is supplied through a blower 11, which is injected in a high speed jet flow from a nozzle 12 to a heat exchanger tube which constitutes the superheater 3. With this, the flow of the exhaust gas passing through the superheater 3 is turned into a turbulence through a disturbance and dust is prevented from adhering to the superheater.