One of an effective global warming countermeasure is CO₂ reduction in urban areas because they emit considerable amount of anthropogenic CO₂. Urban landscape trees have a role of CO₂ absorption by their photosynthesis in urban area. Information of (1) current status of atmospheric CO₂ environment and (2) variation of photosynthetic function in urban landscape trees are essential for effective CO₂ reduction. Seasonal and annual variation of the photosynthesis in a major urban landscape tree species, Ginkgo biloba was investigated from leaf expanding season to leaf fall season from 2014 to 2017 in an urban area in Kyoto city. Concentration and stable carbon isotope composition (δ¹³C) of ambient CO₂ was measured by WS-CRDS gas analyzer. Maximum photosynthetic rate (A_max) of Ginkgo biloba was kept high from spring to summer season and then gradually declined in autumn season. This decline was related to the reductions in photosynthetic biochemistry (V_cmax, J) and stomatal conductance (g_s) of Ginkgo biloba. Concentration and δ¹³C of ambient CO₂ showed diurnal and seasonal variations, which may be influenced by changes in tree photosynthesis. We are now trying to estimate CO₂ source of ambient CO₂ in Kyoto city.