AFP:

The magnitude of their landslide election victory was too much for Japan’s ruling Liberal Democratic Party (LDP), which did not have enough candidates to fill all the seats it would have won in Tokyo.
Prime Minister Junichiro Koizumi’s party had won enough seats to send eight lawmakers to the house in the Tokyo block’s proportional representation system, under which voters choose parties rather than individual candidates.
The ruling party, however, had only seven registered candidates to fill in the slots because so many LDP members managed to win in their districts, rather than through proportional representation.
As a result, the vacant seat went to the left-wing Social Democratic Party, which otherwise would not have elected any lawmakers in Tokyo.