http://www.bbc.com/news/world-asia-35666274
I think there's two main causes of this trend. First is the Japanese economy, which seems to be stuck in a funk since the 1990s and makes it financially straining to have children. Another big one is immigration; Japan is a notorious pain in the ass to immigrate to (I think they don't do dual citizenship, and it seems pretty difficult to assimilate). IIRC, Germany was looking at a declining population until immigration went up (it's now growing), so if Japan can fix that it should help slow the population decline. Not sure how to, though.