Aircraft measurements of the tropospheric N2O concentration were made over Japan during the period from October 1991 to June 1999. The observed values of the N2O concentration showed clear evidence of the secular increase, with mean rates of 0.50 to 0.64 ppbv/yr for selected height intervals of 0--2, 2--4, 4--6, 6--8 km and 8 km-tropopause. Although the seasonality of the N2O concentration was hardly observable throughout the troposphere, interannual variations with periods of about 2 years were clearly found. The average N2O concentrations over the observation period for the above 5 height intervals were almost identical with each other, showing no appreciable vertical gradient of the concentration. This suggests that N2O emissions from the ground surface around Japan are very weak and the emitted N2O is mixed well in the troposphere. ¿ 2001 American Geophysical Union