Japan's next prime minister is Catholic

BBC News: "Members of Japan's ruling party have selected a bluff conservative, Taro Aso, as their new leader, meaning he is almost certain to become the next PM."

He would be the first Catholic PM in Japan's history.

More about Taro Aso on his Wikipedia page. He also loves comics, was quickly profiled by Time, faces a tough future, is a very big departure from previous PMs, and appears to be quite the renaissance man.

Now, here's my question: presuming he wasn't born Catholic (which would surprise me), when/how did he convert? Can anyone find out?