Earlier (about a year ago) I imported some of the countries I found here at geonames into my database. That time I was able to import the geo data into 4 tables for a 4 level hierarchic system of it, which was sorted into what "county" and "state" each area belongs to.
We don't have this information.
Someone once sent me the following link:
http://www.mlit.go.jp/kokudokeikaku/gis/index.html I don't speak Japanese so I don't know whether the information is still there...
The Gisgraphy framework has an importer that can import the Country and the ADM in a tree structure (ADM4->ADM3->ADM2->ADM1). For a given ADM, you can ask for his parent and childs
www.gisgraphy.com is the project site
services.gisgraphy.com is a site where the gisgraphy framework is installed (so you can test it before install it)