In order to examine the role of terrestrial biospheric activity in the seasonal variation of δ18O of atmospheric CO2, we measured δ18O values of CO2 and H2O in the soil and the canopy air in a temperate forest in the northern part of the main island of Japan for the period August 2000--December 2001. It was found that the δ18O value of the soil water near the surface varied seasonally almost in phase with that of precipitation, except for spring. On the other hand, its seasonal variation at 70 cm depth showed a 3-month delay compared to those at 10 and 40 cm depths, owing to slow penetration of precipitation into the soil. The measured δ18O values of CO2 in the soil air at 10, 40, and 70 cm depths agreed well with the results calculated under the assumption that CO2 in the soil air was isotopically equilibrated with its surrounding soil water. On the basis of the measurement results, we derived the seasonally varying oxygen isotopic discrimination factors for respiration and photosynthesis processes. Using these discrimination factors as well as biospheric CO2 fluxes estimated by employing an ecosystem model, we calculated the seasonal variation of δ18O of atmospheric CO2 in the forest canopy. The result showed a good agreement with the observation, indicating that the isotopic contribution of photosynthesis to the atmospheric δ18O was strong in spring and fall, while that of respiration was dominant in summer and winter.