Many attempts have been made to classify the climates of Japan by means of mean values of the official meteorological stations, as, for example, by the January mean temperature. In fact climatic pheno-mena are too small to be revealed by the above-mentioned method, since the stations average 100 kilometre' distance from each other. Besides, regional differences tend to disappear in a calculation of mean values.. Thus this method defines the climatic boundaries too vaguely. The author of this article, in an attempt to avoid these errors, has made daily precipitation charts (scale: 1/1, 500, 000) for a year using all the non-official meteorological stations whose average distance apart is only 15 kilometers. The weather differences were then analysed. The analysis reveals that the most characteristic difference is in air masss prevalence. In Summer the polar front reaches as far north as Hokkaido and moves to the south again in winter. Over North Hokkaido a warm tropical air mass cannot be expected to stay at all during the year. Thus, by means of this difference of air mass prevalence, the climate of Japan is divided into two primary zones. A second division is made by the regional difference of winter precipitation. A third division is created by the concentration of heavy rainfall in certain areas. A detailed study of the daily precipitation chart shows that there is no other important difference of climate in Japan. Using the method described above, the author classifies the climate of Japan into nine categories as below.PPIa PTIa PPIb PTIb — PTIIa PPIIb PTIIb — PTIIIa — PTIIIb PP=Polar air mass both in Summer and Winter. PT=Tropical air mass in Summer and polar air mass in Winter. I=Precipitation in Winter monsoon. II=Transition from I to III. III=No precipitation from the Winter monsoon. a=Heavy rain in cyclone. b=Normal rain in cyclone.