PROBLEM TO BE SOLVED: To provide a watering control method for hydroponic cultivation in which an amount of supplying nutritious water is increased/decreased according to a measured amount of insolation, where cultured crop plants absorb nutritious water appropriately, and basal metabolism which is of the crop plants while they are not insolated is adopted to sufficiently control the quality, yield, and the like.SOLUTION: The watering control method for hydroponic cultivation is characterized by calculating a basal metabolic value Eb (W/m2) by the formula, Eb=[SEa×Vn/(Vd-Vn)]×[106/(60×60×12)] from: an average daytime absorption Vd(L) of crop plants, that is an average value per day for a predetermined term of the previous day and before, where the plants are in an insolated state; an average night time absorption Vn(L) of the crop plants, the value accompanied by their basal metabolism, where the plants are not in an insolated state; and an average integrated insolation amount SEa (MJ/m2). And the method