I called tax office. They had good news. The tax law has been changed from the beginning of this year. So no need pay 35% anymore even if stay less than 6 months. :). Must go to tax office to take a tax card.. And estimate how much are your incomes from Thailand/one year. Then they add it to the salary get from Finland. And the % will be much less than 35. :)
In addition must pay 5.55% insurance from salary for pension. This they will pay back when youll willl be retired little every month when after 65 years old. :)