The objectives of this study are to establish a computation method on the total amount of energy consumption in urban areas, to estimate the quantity of heat that is transferred into wastewater, ultimately going into wastewater treatment plants, and to examine possibility of waste heat recovery. As a result, waste heat transferred to wastewater is 1.9% of the total amount of energy consumption and 37.3% of the amount of energy consumption by electricity only for water heating in midnight, municipal gas, liquefied petroleum gas, liquefied natural gas in Fukuoka and 4.3% of the former and 63.3% of the latter in Kitakyusyu in 1988. The quantity of recoverable waste heat energy with more than 5°C temperature difference from that of ambient atmosphere corresponds to 10% of the total waste heat flux to the wastewater treatment plants. The availability factor of exergy is 1.7% in both cities. Waste heat is available in particular for air conditioning and snow melting on road in winter in Japan.