On the base of an experimental data of two types "East-West type" and "North-South type" of a fully passive solar lumber apparatus, water evaporated from wet lumber was evaluated when the apparatus processing. A calculation method is due to a difference of absolute humidity between an intake air and an exhaust air during the drying process. Two coefficients of the calculation formula were corrected by comparing with values directly measured by an electric-resistance dryness meter. Consequently, it was made clearly that the water evaporated on the drying process is 753〜1320kg(H_2O) at each testing run, using a ratio of water evaporation energy 0.1〜0.4 and correcting factor range of 0.5〜2.3.