PURPOSE: To improve the precision in radiation measurement by a method wherein an insertion part wherein a radiation detector for detecting the radiation of a radioactive material contained in the main body of a container is inserted is provided in the main body of the container along the direction of the center thereof.

CONSTITUTION: An insertion part 1a is formed in the main body 1 of a tubular container the upper end of which is sealed hermetically by a cap 3, so that a neutron detector 4 can be inserted thereinto toward the central part of the container. This insertion part 1a is a well-type cavity formed from the base of the main body 1 toward the central part thereof. A uranium dioxide 2, for instance, is contained in the main body 1, and when the total amount of this uranium dioxide 2 is measured, the detector 4 is inserted into the insertion part 1a. A signal from the detector 4 is connected to a measuring circuit 5. In a radioisotope container provided with the insertion part 1a enabling installation of the detector 4 in the central part in this way, the count of neutrons is large in the central part of the main body 1, while it lessens toward both ends.