After the abolition of post-grant opposition in 2003, invalidation is currently the only possibility for third parties to challenge a granted patent under the Japanese patent system. However, according to a survey among industry, this change to the patent law did not have the expected outcome: whereas, previously, several thousand opposition cases were filed annually, only a few hundred invalidation requests are submitted today.
Possible reasons for this development might be a steady trend of filing third party observations instead, or the high costs and long duration of invalidation suits.
Consequently, the Japan Patent Office (JPO) is looking into reviewing the invalidation system and re-introducing post-grant opposition, possibly in the course of 2014. The re-introduced system should be similar to the previous one, abolished nearly ten years ago.
More information has been released by the JPO in this official document (Japanese only).