To aim at prevention of spontaneous ignition from taking place from a high temperature part by receiving radio wave transmitted from one side of sensor capsule vertically moving inside of powder/granular material by the other side of sensor capsule and detecting a high temperature part in the material from the variation in radio wave intensity pattern.

Air subjected to its pressure regulation in air supplier 10A, 10B is introduced in cylindrical bodies 8A, 8B and sensor capsules 11A, 11B are so controlled as to be maintained in required positions inside of the cylindrical bodies 8A, 8B. Sensor capsules 11A, 11B force cylindrical bodies 8A, 8B to make mutually alternate vertical movement to let the capsule 11A generate radio wave which is allowed to transmit through the inside of coal 3 from a transmission antenna. And this radio wave is received by the antenna of the other side capsule 11B, this wave is amplified by an amplifier, made to flow from this side transmission antenna to a receiving antenna 12B, from which this radio wave is transmitted to a data analyzer 13 and a calculator 15 through a signal line 14. By detection of variation in intensity distribution pattern of this radio wave, generation of a high temperature part is detected.