PURPOSE: To perform rapid and accurate detection of cracked places by detecting the temperature change of good heat conductors of the regions segmented at a fixed spacing by insulators, etc. on the outside of piping by thermocouples.

CONSTITUTION: Hollow insulators 22 ... of convex shape in section are provided at a specified constant spacing on the outside circumference of piping 20 and heat insulating materials 24 ... are filled on the outside circumference of the pipe 20 of the respective regions formed by these insulators 22 ... In addition, good heat conductors 24 ... are wound on the outer side. Hence, if the high temperature fluid in the pipe 20 infiltrates in the region corresponging to the cracked place of the piping, the temperature of the good conductor 24 rises and the cracked place of the piping for power generation of an atomic reactor is detected rapidly and accurately through the thermocouple 23 welded to the good conductor.