What is the difference between an Invalidation Trial and an Opposition in Japan?
An Invalidation Trial adopts “Inter Partes” procedure.
An Opposition adopts “Ex Parte” procedure. Therefore, the involvement of an Opposition Filer is limited mainly to the fling of an Opposition thus triggering the review by the JPO.
Because of this difference, survival through an Opposition does not necessarily mean that the patent was exposed to the keen dispute between conflicting parties.
The law, recognizing this procedural difference and its substantial implication, even if the patent has survived Opposition, filing an Invalidation Trial based on exactly the same reason is not prohibited and such filing can also be done by the very same party who filed an Opposition (and lost).
In order to thoroughly attack the validity of a patent, it is highly recommendable to chose an Invalidation Trial and never chose an Opposition.