Praise be to Allah.
Zakaah is obligatory on four kinds of wealth: agricultural produce (grains, fruits, etc.); freely-grazing livestock and cattle; gold and silver (and their equivalents in modern currencies and bank-notes) – the zakaah on which is 2.5%; and trade goods.
Seeing that the money you have is in the form of paper currency paid to you as a grant from the Japanese government, the easiest way for you with regard to paying zakaah in this case is: wait until after one hijri year has passed, then check your bank and pay 2.5% (twenty-five out of every thousand) from whatever you have. But if you are living hand to mouth, as it were (i.e., spending it all straightaway), and you do not have enough left to reach the level of the nisaab, and a year has passed, then you are not obliged to pay zakaah in this case. And Allaah knows best.