PURPOSE: To obtain an inexpensive and simple sample case for ESR measurement by using a usual ESR cavity resonator as it is, and using a capillary autoclave made of nonmetallic and nonmagnetic material to the sample case.

CONSTITUTION: The cavity resonator is provided outside the sample case of nonmetallic and nonmagnetic autoclave so that the capacity of said resonator is not lost by the damage of the sample case. Then, for making as small as possible the case part for which gaseous pressure in a sample apparatus is applied, the capillary autoclave, a capillary gas introducing pipe, a screw lid, and a stop valve whose dead volume is decreased are used to decrease whole capacity of the apparatus system to the desired volume or below, and so that the capacity of said resonator is not decreased even if the sample apparatus is damaged. In this way, the inexpensive and simple ESR measurement of various samples becomes possible under a prescribed temp. and gaseous pressure by using said resonator and the sample case of existent high temp. ESR apparatus.