Thorn Tree travel forum

1

The MoFA site states that Japan does not have transit visas but instead transit visa waivers. The details are here. But it tells you that you should obtain details at a Japanese consulate.

2

If you just "transit" at the airport, i.e. do not go through immigration, you do not need anything as you will not enter the country (legally speaking).
Otherwise, it seems that you can stay up to three days.

"If the passengers of an airplane or ship that lands at an airport or seaport in Japan wish to enter Japan temporarily for shopping or rest, they can receive special landing permission from the immigration authority at the port of arrival even if they do not possess a visa...

(ii) Permission for landing in transit for departure at a neighboring port
Permission is granted to foreign passengers of an airplane or ship proceeding to another country via Japan who wish to enter Japan from their first port of call and then move to another port of call in a nearby area for departure. This permission is granted when a foreign passenger wishes to depart Japan within three days.
In principle, another port of call in a nearby area is regarded as a port that is under the jurisdiction of the same regional immigration bureau, or the immigration bureau of a neighboring region. (For example, Narita Airport to Haneda Airport or Narita to Nagoya Airport are acceptable, but Narita Airport to Kansai Airport is not allowed.)"

But better check with your travel agency or Japanese embassy...

metaeureka