Japan’s Ministry of Labor has made available an online map of coronavirus cases by prefecture in English. The information includes the total number of confirmed cases by day, the cumulative number of cases, and general information about the infected patients (prefecture, gender, age group). There appears to be a two-day lag in the information being reported.
For information on cases by prefecture, click on the bar for the prefecture, then scroll for data on each case.
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Source: Japan Ministry of Labor