PURPOSE: To carry out high accuracy calibration simply and easily by using a vessel which contains a radiation source of a known radiation density.

CONSTITUTION: In the guide groove 7 of an annular supporter 6 with the annular guide groove 7 supported horizontally by legs 8, a vessel 5 which contains the radiation source of the known radiation density is placed. On the center axis A of the annular supporter 6, a detector 4 consisting of a Geiger-Mueller counter tube to be calibrated is arranged, then the vessel 5 is circled once along the annular groove 7 and the count of the output of the detector 4 is averaged by that one cycle time. By so doing, correspondence between the detection sensitivity of the detector 4 and the known radiation density of the vessel 5 placed at a certain distance from the detector is found and the desired calibration constant can be determined. Therefore, high calibration accuracy can be obtained by a safe and easy operation.