PURPOSE: To make it possible to obtain a thermogram at the arbitrary point of time of an object to be measured, by generating a series of sampling signals corresponding to the cyclic temp. signal of the object to be measured.

CONSTITUTION: Cyclically changed energy is supplied to an object 1 to be measured from an energy supply means 6. The temp. of the object 1 to be measured is detected by a detection part 2 and processed by a signal processing part 3 while the obtained temp. signal is subjected to sampling held by a sampling hold circuit 4 and thereafter inputted to memory 5. The sampling gate signal in the sampling hold circuit 4 is outputted by a sampling gate signal forming circuit 8 and has the predetermined phase relation with energy for cyclically changing the temp. of the object 1 to be measured.