A machine translation from Japanese to English can be done via the Industrial Property Digital Library (IPDL) website. This offers the public access to Japanese patents.

For example if the patent number is JP2010192904-A , follow these steps:

1. Go to http://www.ipdl.inpit.go.jp/homepg_e.ipdl
2. Under Patent & Utility Model click on Patent & Utility Model Gazette DB.
3. In Kind Code type A.
4. In Number type 2010-192904.
5. Click on Search.
6. In the next window clik on the link on the left indicating the patent number.
7. It will start translating for you. Click on the Detail link (pink box at top).
8. Translation will start happening, it may take a bit of time.
9. Click on Detailed description etc.

More examples can be found at http://www.jpaa.or.jp/english/patent/how_to_search.html