100V "rated" wire on 220V supply
TL;DR
The Japanese cable will work, but you may not want to use it.

Why?

As the 110v extension lead is designed to be plugged into 'mains', which experiences 1500v spikes whether on a 100v or 220v system, its insulation will be rated for at least those spikes. So 220v will be no problem.

However, after the fire (caused by some other random electrical thing overheating, or a cooking accident, or your 2 year old playing with matches) the insurance assessors will pick through the debris looking for reasons not to pay your claim. If they find a 100v rated extension lead plugged into a 220v system, even if it obviously didn't cause the fire, they might just make life difficult for you, just when you don't need it to be. 

But check the rating. It's possible that extension leads available in Japan might be rated for 220v, even if Japan uses a 100v system, so that it's saleable in neighbouring territories. If it says 220v on it, then there's not even the insurance problem.