A radiation intensity measuring apparatus is provided for an encapsulated sealed radioactive source for brachytherapy, which is capable of measuring radiation intensity of sources with a cartridge enclosed under sterile conditions. The radiation intensity measuring apparatus includes a radiation intensity measuring device for measuring radiation emitted from a source, a holding device for holding a cartridge, and a moving mechanism for moving the holding device to the radiation intensity measuring device. The moving device includes a guide portion for guiding the movement of the holding device so that the holding device moves along a direction perpendicular to an axial direction of a slit, and a moving portion for moving the holding device so that all the sources loaded in the cartridge pass through a position of the slit in a housing space of a housing portion.