Japan has suffered from COVID-19 with significant mortality, but its prefectural differences are not well investigated. Since the mortality due to COVID-19 was likely to be associated with the number of infected cases, the population density, and the proportion of the elderly population, we tried to detect the outlier prefectures by multiple linear regression analyses. With the use of the Hampel identifier, we found that Hokkaido and Hyogo were the outlier prefectures with higher mortality after adjusting the variables above. Further studies should delineate the causes of these differences.