To improve safety by detecting the level of a silted matter on the inside wall face of a tube, and early detecting the generation of fire.

In this detecting device, the temperature of fluid flowing at the central part of a tube 2 and in the neighborhood of the inside wall face of the tube 2 is detected, and a difference between those temperature is calculated so that the deposition of a foreign matter can be detected according to the change of the difference, and also the temperature change is measured by differentiating the detected value of the fluid temperature so that the generation of fire can be detected. This device is provided with first and second preliminarily heated temperature sensors 4 and 5 which detect a temperature corresponding to radiating amounts changed by a flowing speed, and a detecting part 6 which detects both the deposited situation of the foreign matter and the generation of fire by the differentiation of the measured value. The above mentioned temperature difference means that the flowing speed is made slow due to the deposition of the foreign matter in the neighborhood of the inside wall face. That is, the deposited situation of the foreign matter can be detected by the temperature difference, the temperature change can be exactly detected by the differentiation, and the generation of fire can be easily detected.