PURPOSE: To obtain a measuring instrument which can control the change in the type of a sealed ray source and the angle of a radiation detector simply by setting the radiation detector and the sealed ray source once.

CONSTITUTION: The measuring instrument is provided with a radiation ray detector 15 which is driven so that measurement conditions can be set, a gripping equipment 21 which can be moved to a first position where a sealed ray source 23 opposes the radiation ray detector 15 and radiation rays are measured and a second position where the sealed ray source 23 is attached and detached and then has a gripping part 21a for switching the sealed ray source 23 so that it can be attached and detached, a plurality of holes for storing the sealed ray source 23 on a circular arc passing the second position of the gripping equipment 21, and a storage plate 24 which is rotated with the layout spacing of the storage holes and is elevated so that it contacts or leaves the gripping equipment 21.