山中 慎介 - Left of God closing in on Fierce Eagle's historic record.

So, at this moment, the next candidates are:
Naoya Inoue - WBO 5
Kazuto Ioka - WBA 5 (personaly i would not count him since it started as the regular WBA title)

And that's it. Every other long reigning Japanese champ has had their reign ended in the last couple of years.

 

Other world titlists who are close to double digits are:
Adonis Stevenson - 8
Chayaphon Moonsri - 7

Gennady Golovkin is the only one in double digits but for a long time he was ducked by the WBA champs. And because of that he held the regular title for a long period of his reign.
Daniel Geale vacated back in 2013 so this would put Golovkins defense count at 13.