Discriminating radioactive nuclides of a gamma ray source and nondestructively measuring an image concentration and spatial distribution of a gamma ray by radioactive nuclides. A visualizing apparatus having a container 1 including a gamma ray source 2; a gamma ray detector 7 around the container 1 which detects gamma rays through a collimator 6; a gamma ray detection signal processing device 9 which processes a gamma ray detection signal measuring energy and counted value thereof; an energy discrimination processing device 10 which performs discrimination and intensity analysis of radioactive nuclides by performing spectrum analysis of gamma ray energy and intensity measured for each unit time or position; an imaging calculation processing device 11 which forms images of concentration and space distribution of the gamma ray source for each discriminated radioactive nuclide; and an image display device 12 which performs visual display based on a result of the calculation processing.