With the aim of improving methods for predicting efflorescence of coated fire-retardant-treated（FRT）wood, processes of moisture absorption by coated FRT wood is generalized through moisture absorption experiments under constant temperature and humidity. Pieces of Sugi（Japanese cedar, Cryptomeria japonica D. Don）were treated with phosphoric acid-based chemicals, coated with several types of coating materials, and underwent moisture absorption experiments. The results showed that both the moisture content and the time axis can be made dimensionless assuming a Fickian moisture absorption process. Furthermore, the temperature dependence of absorption speeds seems to follow the Arrhenius equation, thus the diffusion coefficient of the FRT wood and the hygroscopic property of the coating can be integrally evaluated as an apparent diffusion coefficient. The above results clarified the temperature dependence of the moisture absorption rate of coated FRT wood. This implies that increasing temperature is quite effective in accelerating moisture absorption of these materials, and that apparent diffusion coefficients under untested conditions and the time to reach equilibrium moisture content can be extrapolated from other temperature conditions.