High density observations of the air temperature and the long wave radiation were done in Kyoto city for two years. Clear heat islands were observed in clear nights while no air temperature difference between the urban area and the rural area was observed in the daytime. The analysis shows that the main cause of the heat island in Kyoto city is a high thermal inertia in the urban area. The anthropogenic heat release has little effect. An object with small thermal inertia is expected to have high surface temperature under the sunshine, but the surface temperature of the rural area does not show high temperature comparing with that in urban area. The possible reason is also presented.