The volumetric water balance of hypothetical root zone in a plastic house contains the runoff rate through the soil under the plastic wall (horizontal water flow) and the rate of capillary rise through the bottom of the root zone (vertical water flow). To improve the water management of the root zone, the development of an estimating method of both water flows is necessary. In this paper, the characteristics of both water flows and their contribution to the water balance are described through the experimental studies which were done by using the soil containers buried near the plastic wall to measure each water flow. The results obtained are as follows: 1) The total water flow into the root zone of the east bed during the period of 104 days of tomato cultivation, which corresponds to the stage from flowering of third cluster to harvest of sixth cluster, was 119.6 mm of water. This amounts to about 40% of that of irrigation water or to about 30% of the evapotranspiration during the same period. 2) Since the ratio of the vertical water flow to the horizontal one was 1.63, suggesting that most of the water flow was through the deeper soil layer than the root zone. 3) The vertical water flow was closely related to the difference in the soil moisture suction between the root zone and the 40 cm depth under the ground surface outside the house or 80 cm depth under the ground surface inside the house. The relation between these differences in suction when expressed as the mean value for 15-days period resulted in a straight line. 4) On the other hand, the horizontal water flow could not be related to any difference in the soil moisture suction. However, it was shown that the horizontal water flow was connected with the evapotranspiration rate during the same period. 5) A multiple regression equation (Eq. 6) for the estimation of total water flow into the root zone was presented, which was composed of the difference in the soil moisture suctions and the evapotranspiration rate.