By statistical treatment of air-pressure data at the summit of Int. Fuji, an attempt was made to discuss the year by year fluctuations in variability of the magnitude of North Pacific anticyclone during 1941-1985 in the relation to characteristic of summer. The air-pressure, at the height of the observation station (3, 772m), generally varies from 625 mb in winter to 650 mb in summer. The properties of summer is well discribed by a dominancy of the North Pacific anticyclone over Japan.<br> From the 45-year mean annual variation of the air-pressure at Mt. Fuji, it can be seen that the period from July through September, i.e., period including the midsummer, coincides with the one when the air-pressure is above 645 mb. Accordingly, the value of 645 mb is adopted as the representative of the dominancy of the North Pacific anticyclone in the present study. Then following four indices are defined. Nam ely, (A) first day of period above 645 mb in air-pressure at the summit of Mt. Fuji, (B) last day of the period, (C) total number of days for the period and (D) accumulated air-pressure above 645 mb.<br> In addition, (E) ending day of Baiu and (F) total number of days of south-high-north-low surface pressure pattern (summer-type pressure pattern) are discriminative for summer con-ditions. Correlation coefficients among the time series (A)-(F) were calculated. The correlation coefficients between (D) and (E) and between (D) and (F) were -0.556 and 0.712 respectively. The accumulated air-pressure is recognized as a significant indication for summer conditions.<br> Variation of the value of index (D) was represented to be at maximum stages in the beginning of the analyzed period (1942-4949), and in the middle (1960-1962), and at a minimum stage during the decade of 1950-1959. After 1963, the fluctuation of the index has been large (see Fig. 5). It indicates that the climate of summer has recently tended to be unstable. According to agricultural data, cold weather damages over Japan occurred twelve times throughout the analyzed period. All the damaged years except 1945, 1964 and 1969 were clearly in accordance with the negative anomaly of the accumulated air-pressure values.<br> In 1980, for example, unusual low temperatures were due to northeasterly cold air advecR tion following the blocking activities over the Sea of Okhotsk. The magnitude of North Pacific anticyclone was remarkably weak and the value of the accumulated air-pressure above 645 mb was reduced to 104. 1 mb day, only 55 percent of the mean value. We can thus investigate a variation of summer property using the indices mentioned above for analyses of the climatic change.