As for rice cultivation in cool region of Japan, it is very important to get heading date within a period which could avoid both destructive damage and inhibition of ripenning caused by cool air temperature. In order to evaluate more safe plan for mechanical rice cultivation in Hokkaido, the quantitative effect of changeable natural air temperature on the growth of rice plant was investigated by the method of accumlated heat unit in effective degrees. The results are as followes. 1) Effective aero-heat unit (θ) was given to each classfied air temperature into 2°C. This was introduced from the relation between each temperature (Ti) and days for increase by 0.2 leaf age in rice plant at Ti (Di). This relation was obtained experimentally by measuring elongation of 8th leaves with 3 rice varieties, "Kitakogane", "Kitahikari" and "Matsumae" in the air temperature controlled growth chamber. 2) Concerning 9 varieties of 8 cropping seasons transplanted young seedlings, 3 index namely days (ΣD), sumed total of diurnal mean temperature (ΣT) and accumlated aero-heat unit in effective degrees (Σθ) from transplanting to heading were calculated. Coeffcients of variance of Σθ among 8 cropping seasons was considerably smaller than that of ΣT and ΣD. 3) Same tendency was also obtained in different years, localities and cropping seasons in Hokkaido districts. Therefore, Σθ could be considered as an useful thermal index to estimate heading date in paddy of young seedling transplanting cultivation in cool region.