Although a spouted bed with a draft tube is widely used in pharmaceutical particle coating to modify the release characteristics of various medicines, the coating conditions and pharmaceutical formulations are determined mostly based on experience and experiments. In the present work, we theoretically analyzed the drying process of particles by applying a cell model, which is defined as an average air volume occupied by a single seed particle. We established material balance and heat balance equations in the cell, and analyzed the drying process using evaporation flux measured by experiments. Furthermore, experiments were carried out to measure the drying process of seed particles in laboratory-scale equipment. As a result, it was found that calculated tendencies of increasing or decreasing water content of seed particles are in good agreement with the experimental data. The parameters controlling the drying process of seed particles were derived by the present analysis and the influences of these parameters discussed.