The Ministry of the Environment (ME) of Japan adopted WECPNL as the indicator of aircraft noise exposure, which was calculated on the average of maximum level of noise events on power basis and the number of noise events a day. Taking the differences between the flight operations in civil and military airports into account, the Defense Facilities Administration Agency (DFAA) proposed slightly different method for calculation of WECPNL as compared to the one proposed by the ME. The main difference between the two measurement methods is that the DFAA method employs 90 percentile value per annum as the standard number of daily flights while the ME method uses mean value. This paper analyses the differences between the two methods on the basis of the measurements around 6 military airfields in Japan. The calculation with the DFAA method showed 3 to 5 units higher WECPNL than that with the ME method for the same noise exposure. Additionally, larger differences were found at some measuring sites where noise from engine tuning was mainly observed. It was concluded that the application of the ME method provides slight underestimation of aircraft noise exposure from the point of view of community impact assessment.