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Column-averaged volume mixing ratios of carbon dioxide (<i>X</i>_CO₂) during the period from January 2007 to May 2008 over Tsukuba, Japan, were derived using CO₂ concentrations measured by Continuous CO₂ Measuring Equipment (CME). The CMEs were installed on Japan Airlines Corporation (JAL) commercial airliners, which frequently fly to and from Narita Airport. It was assumed that CO₂ profiles over Tsukuba and Narita are the same. CO₂ profile data for 493 flights on clear-sky days were analyzed in order to calculate <i>X</i>_CO₂ with one of two ancillary datasets: "Tsukuba observational" data (rawinsonde and meteorological tower), or "global" forecast/reanalysis and climatological data (NCEP and CIRA-86). The amplitude of the seasonal variation of <i>X</i>_CO₂ using the ancillary data measured in Tsukuba (<i>X</i>_CO₂ (Tsukuba observational)) was determined by a least squares fit using a harmonic function to roughly evaluate the seasonal variation over Tsukuba. The highest and lowest values of the obtained fitted curve in 2007 for <i>X</i>_CO₂ (Tsukuba observational) were 386.4 &plusmn; 1.0 and 381.7 &plusmn; 1.0 ppm in May and September, respectively, where the errors represent 1 standard deviation of the fit residuals. The dependence of <i>X</i>_CO₂ on the type of ancillary dataset was evaluated. The average difference between <i>X</i>_CO₂ from global climatological data, <i>X</i>_CO₂ (global), and <i>X</i>_CO₂ (Tsukuba observational), i.e., the bias of <i>X</i>_CO₂ (global) based on <i>X</i>_CO₂ (Tsukuba observational), was found to be −0.621 ppm with a standard deviation of 0.682 ppm. The uncertainty of <i>X</i>_CO₂ (global) based on <i>X</i>_CO₂ (Tsukuba observational) was estimated to be 0.922 ppm. This small uncertainty relative to the GOSAT precision suggests that calculating <i>X</i>_CO₂ using data from airliners and global climatological data can be applied to the validation of GOSAT products for <i>X</i>_CO₂ over airports worldwide.