You can reach Mikkeli easiest with Train, which takes only 2 h 30 min from Helsinki. There are daily several Train connections from Helsinki to Mikkeli; timetables and prices
Otherwise you can fly from Helsinki to Savonlinna with Finnair and take bus from Savonlinna to Mikkeli. The bus ride from Savonlinna to Mikkeli takes about 1 h 30 min.; timetables and prices
The Finnish capital Helsinki is easily accessible, no matter from where you are travelling from, and is served by many international flights. If you want to travel with your own car, you can reach Helsinki via ferry from Tallinn, Stockholm, Travemünde and Rostock.
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