Atmospheric CO2 concentration and its isotopes have been measured at the Takayama site in central Japan since 1994, together with δ18O in precipitation since 2002, to examine their temporal variations in a cool-temperate deciduous forest environment strongly influenced by the Asian monsoon. The CO2 concentration and δ13C at the observational site show prominent seasonal variations accompanied by a secular trend, suggesting that the average seasonal cycle is caused mainly by a seasonal-dependent CO2 exchange with C3 plants. However, the variation in summer is found to be related to CO2 exchange with a significantly heavier δ13C signal compared to the other seasons, indicating some influence at the Takayama site of atmospheric transport of CO2 impacted by exchange with C4 plants from upstream regions. Compared with CO2 and δ13C, δ18O measurements in atmospheric CO2 are scattered, showing no clear seasonal variation. From late spring to early fall, significant year-to-year differences in the variation of δ18O are observed, with low and high values associated with rainy and sunny summer conditions, respectively. Comparisons with data from other northern midlatitude sites suggest that the rapid decrease of δ18O seen at Takayama in the early summer of a rainy summer year is characteristic of the Asian monsoon region. On the other hand, the characteristic increase in δ18O observed from winter to spring is likely caused by long-range transport of air with enriched δ18O.