Measures by the government of Japan - Japan National Tourism Organization
Japan Airlines: Travel Alert - Information regarding COVID-19
The COVID-19 pandemic was first confirmed to have spread to Japan in January 2020.  Cases have been recorded in each of the 47 prefectures except for Iwate. The country has seen over 10,000 confirmed cases.
The Japanese government confirmed its first outbreak of the coronavirus disease 2019 (COVID-19) in the country on 16 January 2020 in a resident of Kanagawa Prefecture who had returned from Wuhan, China. This was followed by a second outbreak that was introduced by travellers and returnees from Europe and the United States between 11 March and 23 March. According to the National Institute of Infectious Diseases, the majority of viruses spreading in Japan derive from the European type while those of the Wuhan type have been disappearing since March. Source: Wikipedia 18-5-2010