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If you want to visit Sapporo, Japan.
Submit necessary COVID-19 negative certification issued within 3 days.
The days counts from sample-taken-day.
Also, if you want to stay in Japan, you must have smartphone to be checked by Japan government in self quarantine days.

COVID-19 test must be proceeded to all visitors to Japan in airport.
Self quarantine must be followed all incoming passengers abroad.
within this period, public transportations are prohibited.