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Summary : For the purpose of easy judge by growers of water stress and irrigation time in fruit trees, an infiltration method using kerosene liquid only was examined for several deciduous fruit trees （five fruit tree species and nine cultivars）. Close and quantitative relations were found between the stomatal pore areas measured by an image processing analysis method and the infiltration times. From the experiments using the detached leaves in which the infiltration time was compared under different air temperatures, a calibration method was devised to correct measurement errors which were brought about by differences in air temperature. Changes of water vapor diffusive resistances （Rls） and the corrected infiltration times were investigated on both the potted fruit trees and the detached leaves under water deficits. Approximate leaf water potential （ψl） in grapes, a persimmon, cherries, apples and pears, when the Rl began to rise suddenly, was －1.2MPa, －1.4MPa, －1.8～－2MPa, －1.7 ～－1.9MPa and －2.1～－2.5MPa, respectively. Infiltration times as photosynthesis declines at various degrees were estimated from the relationships between ψls, Rls and the corrected infiltration time in each cultivar, and proposed as indices of the irrigation commencing time. The amplification of the index was procured in a case of substituting liquid paraffin when the infiltration time using kerosene passed in the shortness.