The Ogasawara Islands are located in latitude 27°N belong to the subtropical region. In these islands, tomatoes are usually sown late summer and seedlings are transplanted at fall in the field. However, the early yield is low remarkably by the poor fruit set of lower clusters. To find the relation between low fruit set percentages and meteorological factors, experiments were carried out. 1. In the field, the fruit set percentages on each flowering date were observed. The result of investigation on the relation between fruit set percentages and environmental factors, showed that the high amounts of solar radiation were effective, on the other hand, daily maximum temperatures were ineffective for poor fruit set. It was suggested, also, that high Water Suturation Deficit (W.S.D.) in leaves caused by high solar radiation decreased fruit set percentages strongly. 2. Twenty-four plants were grown in Wagner's pots. Half of them were irrigated every day, however, the remainder did not water one day during the experiment, to be suffered water stress. The ratio of fruit set percentage on each flowering day was indicated as latter to former. Consequently, this ratio on the non-watered day dropped remarkably, and also dropped on the previous day. However, the ratio rose on the next day conversely. 3. Fifty-four plants grown in the field were divided between two plots. One was shade-treated plot: the plants belong in the plot were shaded a couple of days two times. The other was untreated plot: the plants belong to the plot were not shaded. Over 11 days, the daily W.S.D. at 13:00, and the fruit set percentages on each flowering day were observed. Consequently, the daily W.S.D. ratios of shade-treated plot to un-treated plot, and the daily fruit set percentage ratios of shade-treated plot to un-treated plot, were inversely related. 4. Photosynthetic rate, solar radiation, and W.S.D. at 13:00 were daily correlated in direct proportion to each other. Accordingly, reduction of photosynthetic rate was not observed at the water stressed day. 5. Thus, fruit set percentages on each flowering day dropped according to the increase in daily plant water stress. However, the physiological causes were not shown in the lack of photosynthetic rates, it should be studied as the subjects of hormones or matter translation.