This paper presents the biological effects of rare earth elements (REEs) on respiratory organs investigated by the research groups of National Institute of Industrial Health and others. REEs are such important components of current industry that the elucidation of the biological effects and environmental effects is a matter of course. In our study, animal experiments have been carried out employing both of intratracheal administration and inhalation exposure into rats for the sample materials: Y2O3, La2O3, Nd2O3, CeO2, and Sm2O3, in which CeO2 included two kinds of samples of different particle sizes. The acute toxicity of rare earth oxides was weak, but the pulmonary effects continued for a long time. The typical changes of lung tissues were pathologically observed as pulmonary alveolar proteinosis, granuloma and pulmonary fibrosis. The rats to which were injected CeO2 of coarse particle did not show any pulmonary lesion, but those of fine particles showed lung lesion at the same level as the other rare earth oxides. As a conclusion, subchronical pulmonary effects from rare earth oxides was observed to respiratory organs although it did not cause serious acute toxicity. Therefore, countermeasures, such as wearing respiratory mask, would be basically necessary for the dust workers using REEs.