As of today (April 1st, 2015), the Post Grant Opposition system comes into effect.
An Opposition can be filed against any Japanese patents published on or after April 1st, 2015. Japanese Patent Nos. 5690984 to 5695400 were published today. Therefore, any Japanese patent having a patent number of 5690984 or more could be opposed.
The Opposition must be filed within six months of the publication date of the patent (Article 113). Attention needs to be paid to the deadline since the filing period is non-extendable. Any third party can file an Opposition (Article 113). A third party opposing a published patent may be a "straw man", but an anonymous opposition is inadmissible (Article 115(1)).