For the last decade, the annual number of extremely hot days, defined as over 35 degrees Celsius by Japan Meteorological Agency, has increased throughout Japan. Tajimi, located in the central part of Japan and near Nagoya (the third biggest city in Japan), reached the record high temperature in Japan of 40.9 °C on August 16 of 2007. Tajimi is not a large city, but the frequency of extremely hot days is higher than in Nagoya and in Tokyo. The increasing health damage and cooling energy consumption associated with extremely hot days are very serious. Factors such as the effect of urban heat islands in Nagoya and the topographic basin effect are assumed to influence extremely hot days in Tajimi. However, the contribution of each of these factors is not clearly understood and the physical processes of extremely hot days are complicated. In this study, we carried out a sensitivity analysis of extremely hot days in Tajimi by using a regional atmospheric model, WRF (Weather Research and Forecasting) (Skamarock et al., 2008). We used the WRF modeling system with the ARW dynamic solver to investigate the contribution of each of the factors of extremely hot days in Tajimi. To evaluate urban thermal environments such as extremely hot days in summer, short-term predictions are not adequate because they often contain uncertainties. Therefore, long-term predictions are necessary and so we conducted long-term predictions of the thermal environment of the area including Tajimi and Nagoya (The horizontal size of the smallest computational domain is 120 km × 120 km) for each August from 2006 to 2010. We performed a sensitivity analysis of four factors that are thought to affect extremely hot days in Tajimi: (1) The effect of urban heat islands of Tajimi itself, (2) The effect of urban heat islands of Nagoya, (3) The topographic basin effect (the effect of mountains) around Tajimi, (4) The effect of the breeze from a big lake (Lake Biwa). Although Tajimi is a small city, Nagoya located to its southwest is one of the fastest growing cities in Japan with an increase in the annual-mean surface air temperature for the last 100 years of 2.9 °C. Tajimi is also surrounded by mountains and the air temperature in such basins is apt to be high. The breeze from Lake Biwa, the biggest lake in Japan located to the west of Tajimi, is thought to moderate the extremely hot days there. The relative contribution of these four factors was estimated by testing five cases. First, present thermal environment was predicted. Second, the urban area of Tajimi was replaced by a grassland area. Third, the urban area of Nagoya was replaced by a grassland area. Fourth, the mountains around Tajimi were removed. Fifth, the water area of Lake Biwa was replaced by a grassland area. We evaluated the difference in the air temperature among the five cases and estimated the relative contribution of the factors on extremely hot days in Tajimi. We found the effect of urban heat islands of Tajimi on the extremely hot days was the most significant in contrast to the surrounding topographical effects which were insignificant. The contribution of the urban heat islands of Nagoya was dependent on the year; however each year, it was larger than the topographical effects.