PURPOSE: To be able to detect local temperature rise in the initial stage and make it possible to take a fire extinguishing action in the early stage by installing a plurality of sound wave transmitters and receivers in the exhaust gas duct and calculating the temperature distribution in the cross section of the duct from the speed distribution of sound waves to judge whether or not the temperature has risen.

CONSTITUTION: To a regeneration type air preacher consisting of, for example, an exhaust gas duct 14, combustion air duct 15, heating element 17, and motor 16 installed in an exhaust gas smoke tube of a boiler which is restarted after hot banking is carried out sound wave transmitters 30 and 31 and receivers 32 and 33, relay for transmission 34, A/D converter, amplifier 36, computer 37 and display device 35 are added. And transmission times are measured for a plurality of sound waver transmission routes that are formed by the combinations of the transmitters and receivers. From those transmission times a sound speed distribution is sought in a cross section of the fluid flow routes. From the sound speed distribution the temperature distribution in a cross section of the fluid flow route is calculated to judge whether or not there is a rise in the temperature in an air preheater or nitrate removal device and the results of the judgement are displayed.