Hi Ryosuke, 'Keen' is not specific to either American or British English. It is used in both, but it is used rarely. English speakers in North America often use it because it is old, and so it can be used in sarcasm if we want to make an funny understatement by using very formal language to express a very simple idea. For example: I would walk on the ice, but I am not very keen on falling and hurting myself. Or, I would drink that beer, but I am not too keen on getting sick. It is very rarely used in its basic form except in certain phrases like 'a keen student' or 'a keen interest.' We do not use it to invent new phrases the way we use many more common words.