Misasa Spa is well known as the most radioactive hot springs in Japan (maximum Rn content: 2330×10(-10) curie/l) and it has been applied to the management of neuralgia, rheumatic diseases, gastrointestinal disorders, etc. In this paper the climatological factors in Misasa Spa during the last decade (1956～1965) are summarized. The main features are as following: 1. The atmospheric temperature attains highest above 30℃ during the last ten days in July and the first ten days in August. In winter the temperature goes down below 0℃. during the last ten days in January and the first ten days in February. Owing to the high humidity, however, it is not felt so cold. 2. The amount of precipitation is more than 2,000 mm or so through the year. 3. The westwards wind is prevailing. The wind grade of 2～3 are observed most frequently. 4. Approximately 150 days of the year are clear fairly clear. 5. It is proved unexpectedly that it is rich in or the ultraviolet radiation even in winter. The results pointed above may suggest that the therapeutical period is optimal between March and October.