Urbanization causes the so-called heat island phenomenon that raises urban atmospheric temperature higher than that in the suburbs. The phenomenon leads to a decrease in heating load and an increase in cooling load in the urban buildings. In this paper, the effect of the heat island phenomenon on the heat load of buildings was analysed by year-to-year changes in the heat load both in the urban and suburban areas. Tokyo was selected as the target urban area and Choshi, as a less urbanized city, among 30 stations located in a range of about 100 km both east-west and north-south centering on Tokyo. Investigation on the relationship between the changes in heat load during the past 100 years confirmed that • The heating load of Choshi has decreased about 20% since 1900, whereas the heating load of Tokyo has decreased about 40% in the same period of time. Also • The cooling load of Tokyo has increased about 20% since 1900, but the similar increase can be seen in that of Choshi.