US & Japan Social Security/Nenkin and taxes

I'm trying to make sense of the US-Japan tax treaty provisions for both US Social Security and Japanese nenkin. I'm a US citizen and permanent resident in Japan, and after I retire I hope to stay in Japan. I'll get Japanese nenkin plus some US Social Security (reduced considerably due to the WEP).

I understand that I must report both amounts to the IRS for US tax purposes, and that I'll probably have to pay US tax on the Japanese nenkin and also on a part of my Social Security. Bummer...

What about the Japan tax situation? I read that I must report the Japanese nenkin in Japan and that I can get an exclusion from taxation in Japan on much of it (700,000 yen exclusion before age 65 and 1,200,000 yen exclusion after age 65). But do I also have to report the US Social Security in Japan if I'm already reporting it in the US and probably paying some US tax on it to the IRS? Does the tax treaty have anything specific to say about this? If anyone knows for sure, please help. I've been looking, but haven't been able to find a clear explanation.

I intend to go to the local tax office to inquire, but before I do that I'd like to find out what I can and be prepared.