Sap flow techniques are practical tools for estimating tree transpiration. Though many previous studies using sap flow techniques did not consider azimuthal variations of sap flux density (F(d)) on xylem trunk to estimate tree transpiration, a few studies reported that ignoring the azimuthal variations in F(d) could cause large errors in tree transpiration estimates for some tree species. Therefore, examining azimuthal variations in F(d) for major plantation tree species is critical for estimating tree transpiration. Using the thermal dissipation method, we examined azimuthal variations in F(d) in six trees of Japanese cypress Chamaecyparis obtusa (Sieb. et Zucc.) Endl., which is one of the most common plantation tree species in Japan. We recorded considerable variations among F(d) at four different azimuthal directions. The F(d) value for one aspect was more than 100% larger than those for the other aspects. We calculated differences between tree transpiration estimates based on F(d) for one to three azimuthal directions and those based on F(d) for four aspects. The differences relative to tree transpiration estimates based on F(d) for four aspects were typically 30, 20, and 10% in accordance with the F(d) for one, two, and three measurement aspects, respectively. This finding indicates that ignoring azimuthal variations could cause large errors in tree transpiration estimates for Japanese cypress.