Tax: Declaring savings a/c income

I have relatively small overseas savings (maybe around $20,000 in savings accounts) and as interest rates are low, they hardly make me rich.

However, over the next few years I expect income from the interest to begin to grow, albeit still in very small amounts.

My questions are:

1. Do I declare ALL interest? For example, once account I have shows barely 10cents interest since newly opened recently.

2. HOW do I declare this on the tax return form? Do I just write the amount of interest earned in the appropriate section (converted to yen, I assume) and leave it there, or do I need to include documentary evidence? In this case it could only be a bank statement or print-out of online a/c statement. Or do I need some other kind of document?

I want to keep myself clear with the tax authorities, so any advice appreciated.

Are you talking about American 1040 or japanese taxes?

Japan only (but declaring all worldwide income).