There is a strong pressing to clarify the effect of thinning coniferous plantations on components of the forest water cycle in Japan. This study evaluates changes in canopy transpiration (E) due to thinning of a Cryptomeria japonica plantation, the most common type of plantation in Japan. Using E derived with the sap-flux method, we modeled canopy conductance (G_c) for periods before thinning and after thinning with the input of the vapor pressure deficit separately. We hypothetically calculated E values using these G_c models, respectively, under the same meteorological conditions. The ratio of E estimated using the G_c model after thinning to that before thinning was 56%. This value was comparable to the ratio of the total sapwood area for the stand (A_s) before thinning to that after thinning (63%). This suggests the possibility of predicting the relative change in E due to thinning using data for the mean diameter at breast height and stem density for the target stand before and after thinning, which are readily available for most C. japonica plantations in Japan and are used to predict A_s.