Long-term changes of urban temperature was examined by using hourly or four-hourly data at 42 stations in Japan. The magnitude of temperature change (T') was defined by the difference between averages for two periods: one from 1891 to 1928 and the other from 1977 to 1994. At many stations, T' values are larger at nighttime than in the daytime, and are nearly constant between a few hours after sunset and sunrise. Moreover, nighttime T' values are nearly equal to the change of daily minimum temperature (T'_min). However, some stations in the northern part of Japan show larger values of T'_min than T' at each fixed time, with a difference of 1°C or more at stations in inland of Hokkaido. This indicates a decrease in nocturnal temperature fluctuation, and is likely to be explained by a weakening of the surface inversion due to urban growth.