[Wasabista]

Okay here's a f'rinstance of what I'm talking about.
I just returned to Thailand from Japan today. At Narita I was offered this rate:
1 baht = ¥3.15
At Suvarnabhumi Airport in Bangkok I exchanged at this rate:
¥1 = 0.3621
Inverting this, we get:
1/.3621 = 2.762

So one baht would have cost me ¥3.15 if I'd bought in Japan, but I waited till I got to Bangkok and paid only ¥2.76 for the same one baht.

So change your money before you arrive in Japan, not after.