The aim of this study was to apply the transpiration integrated model proposed by Sago et al. (2011c) to the simulation of the day-to-day dynamics of root ion absorption of tomato plants in soilless culture. Quantitative data on root ion absorption during the daytime and nighttime were obtained using a nutrient film technique (NFT) soilless culture system in which tomato plants were cultivated, and the data were analyzed using the transpiration integrated model. The identified model parameters could represent the characteristics of root ion absorption. The day-to-day dynamics of root ion absorption were simulated reliably in the daytime, but the simulation of the nighttime ion absorption was difficult. Nevertheless, the nighttime ion absorption accounted for a small portion of the daily ion absorption, and the transpiration integrated model was found to be effective for evaluating the root ion absorption over the entire day. This model is expected to be applicable to the simulation of root ion absorption in NFT soil-less culture for sustainable nutrient management.