Aircraft measurements of atmospheric CO2 concentration over Japan have been initiated in January 1979 at the Upper Atmosphere Research Laboratory of the Tohoku University. An average annual increase of CO2 concentration during a period from the beginning to April 1981 is estimated to be 1.5 ppmv/year. Yearly average values for the concentration decrease with increasing height above ground, rapidly in the lowest layer of the tropopsphere, and rather slightly in the layer above it, showing that the ground acts as a CO2 source. The amplitude of the seasonal variation decreases markedly with elevation. The maximum occurs early in May and the minimum early in August at low levels. The phase shift of the seasonal variation between the lowestmost and uppermost parts of the troposphere is about 1 month. Relations between irregular variations in CO2 concentration and characteristic features of weather systems are discussed.