On March 6th, 2011, Yahoo Japan released map data to OSMFJ. The data was originally held by the ALPS corporation, which merged with Yahoo Japan. We can import the data provided by Yahoo Japan in off-line to OSMFJ. Note that this Yahoo Japan Alps data is not the map data you can see and use at yahoo.com or yahoo.co.jp. The maps at yahoo.com and yahoo.co.jp are not permitted for use at OSM.
Following the March 11th, 2011 tsunami, much mapping was needed in NE Japan (Aomori, Iwate, Miyagi, and Fukushima). Much of that work was completed in the spring and summer of 2011. For many other areas in Japan, though, there's still a lot of data yet to be imported. See the Japanese version of this page for a list of what has been imported where. And, if you do an import, please update that page (or leave a comment here and someone who speaks Japanese will).
It is important not to remove pre-existing map data — the ALPS data is not perfect, and what is already in place, though limited in many cases, is presumably quite accurate, since in large part it is based on GPS data. In areas with high resolution aerial photography, you may be able to compare pre-existing data with ALPS data and decide which seems more accurate. So, first import the ALPS data, and then merge it with the existing data. For the source tag, please use "YahooJapan/ALPSMAP".
The easiest places to import are those for which there is currently almost no work done — this is much simpler than places where a lot of merging and combining of ways is required. Also, smaller areas take less time than larger areas. In Japanese political terms, the most populated areas (meaning the most roads, typically) are cities (市), followed by towns (町), and the least populated (and thus, typically, the fewest roads) are are villages (村).
The map data can be downloaded at http://openstreetmap.jp/YahooAlps/roads/.