Monitoring of environmental ionizing radiation and radioisotopes in Okayama city and the vicinity was carried out from the point of view of the external radiation from the environmental radiation dose. Although the fluctuation of monthly detected radiation dose throughout the year was rather low at each observation point, the mean value of dose equivalent was varied between the place of observation. From the observation of the distribution of environmental radiation dose in Okayama city, it was assumed that its distribution was well corresponded with the distribution of igneous rocks. The concentrations of radioisotopes in various kinds of rocks corrected in Okayama city and its vicinity were determined. It became apparent that high concentrations of radioisotopes, such as (134)Cs, (40)K, (226)Ra and (228)Th, were detected in igneous rocks, such as granite, gabbro and rhyolitic tuff, although the contents in sedimentary rocks, such as pelite, schist, psammite and limestone, were rather low. This result was in good agreement with that of the distribution of radiation dose in Okayama city.