Milky Way Planner has pages for 4 dark sky places in Japan: 2 parks, 1 community and 1 urban night sky place. Their Bortle classes run from 2 to 6. Each page has the Bortle class, the best months for the Milky Way and a free visibility calendar.
Community · Bortle 6
Park · Bortle 2
Park · Bortle 4
Urban night sky place · Bortle 5