The invention provides a radiation intensity measuring apparatus for each of small sealed radiation sources for cancer therapy that is capable of measuring multiple cartridges. The apparatus includes a device for setting up multiple cartridges and a device for measuring the radiation intensities of radiation passing through a narrow slit. Radiation is emitted from multiple radiation sources and the radiation measurement device moves simultaneously together with the slit along with the side of cartridge holding device while scanning the radiation intensities. Included is a radiation detection scanning device for measuring radiation intensities for each of the radiation sources passing through the narrow slit, and a slit shielding system for restricting radiation originated originating from both the side neighboring radiation sources. The moving device is configured move the radiation measurement device along the direction in the side of cartridge where multiple radiation sources are lined up close to each other.