Measurements of atmospheric CO₂ were made continuously at a height of 34 m in Sapporo (lat 43.1°N, long 141.3°E), located in northern Japan, from November 2004 to December 2005. Air samples for measurements of atmospheric CO, CH₄, and CO₂ were also taken in Sapporo and Ishikari Hama (lat 43.3°N, long 141.4°E), facing the Japan Sea, at 10- to 14-day intervals during the same period. In Sapporo, the atmospheric CO₂ data showed clear diurnal and seasonal variations. During the diurnal variation, the maximum CO₂ concentration occurred in the morning and the broad minimum in the daytime. The daily mean value of atmospheric CO₂ in the daytime (11-16 JST) was at a maximum in winter to spring and a minimum in summer. From December to February, the daytime atmospheric CO₂ concentration was about 8-13 ppm higher than that in the background air at Ishikari Hama; from July to August, it was nearly equal to that of the background air. In winter, the atmospheric CO concentrations in Ishikari Hama and Sapporo showed a good correlation with atmospheric CO₂ (10 ppb CO/ppm CO₂, r = 0.82). However, atmospheric CH₄ remained fairly constant against CO₂ change. In summer, CO₂ flux by the photosynthesis of terrestrial vegetation between 11 and 16 JST was nearly equal to that of area-averaged CO₂ emission (between Ishikari Hama and Sapporo) due to human activities, which was estimated to be 11 μmol m^-2s^-1 in the catchment area.