Automobile observations of air temperature distribution were conducted in order to investigate nocturnal distribution of temperature and its causal factors in Tosa City, Kochi Prefecture. The results obtained in this study are as follows : (1) From October to July, an island-shaped high temperature region was formed along the National Road N0. 56, which runs through the central part of the city area from east to west. 0n the other hand, the paddy field zone around the city area had a lower temperature than the city area. (2) The difference of temperature of the central part of the city area and a suburban part was 3.5℃ at the maximum. This value is about 0.5℃ lower than that observed in Kochi City and in Nankoku City. (3) As a result of multiple regression analysis, buildings, fields, and the distance from the National Road N0.56 are found to be major factors for temperature distribution during the nonirrigating term of the paddy fields. 0n the other hand, buildings, paved roads, parking lots, and the distance from the National Road No.56 affect temperature distribution during the irrigation term of the paddy fields. As this reason, it is suspected that the radiative cooling of night causes the temperature fall of suburban areas greatly during the non-irrigating term of the paddy fields, though its influence is small during the irrigation term of the paddy fields. And during the irrigation term of the paddy fields, it is possible that the discharge of heat from concrete or asphalt makes the city region hotter than the suburban areas.