The environmental noise at an arterial road side (Route 1) has been measured in Nagoya City since 1974. In this measurement, the values of LAeq, LA5, LA50 and LA95 were obtained for every one hour. In this study, the effect of observation time on the frequency distribution of LAeq was investigated using the data of LAeq, 1h measured in 1990. The results are summarized as follows. (1) The frequency distribution of hourly LAeq, 1h tends to compact unimodal distribution, if the samples are classified by weekdays or holidays and subdivided into each time section of the days. (2) The dispersion of distribution in nighttime is, however, much larger comparing with that in daytime. Therefore we need longer observation time and the appropriate averaging to get a stable LAeq in nighttime.