In ancient Japan when medical science and science were not developed, the names of diseases left in the documentary records were not many because of the lack recognition of diseases. People in ancient Japan depended much upon the folk beliefs in the recognition of diseases and they depended upon the folk beliefs rather than the doctors or medicines in the matter of treatment. Although the names of diseases expressed in the documentary records were not a few, even in the ancient times of the lack recognition diseases, 'Mononoke' which could be applied to any kind of diseases stroke the terrors into the peoples' hearts. 'Mononoke' was widely used just like the pronoun of diseases from a slight symptom of disease to a fatal disease or a contagious disease. Even in ancient Japan there were doctors and many kinds of medicines, but people who recognized the cause of a disease as 'Mononoke', a phenomenon of nature, believed that in order to treat a disease to conquer 'Mononoke' was the most effective method through the spells and prayers of ascetics or monks. When there happened to be a patient, he looked for an ascetic or a monk instead of a doctor or medicines. Therefore, people in ancient Japan thought the spells and prayers the best methods to treat diseases.