National Diet Library now enters "Linked Open Data Challenge Japan" as one of the partners who provide data. It is the Japan's first contest which honors efforts for creating open data and use of data.

Metadata of the sounds, videos, photos and documents in HINAGIKU is available.
 
Linked Open Data Challenge Japan 2013 (Japanese)
 
How to use data or platform of partners > National Diet Library (Japanese)
 
How to enter (Japanese)