As soon as you go outside of customs you will see a place where they are exchanging currency for yen.
The OP wrote "300 hundred dollars".
Yes, if OP meant 300 $100 bills, that's $30,000.
But if OP meant 3 hundred dollars, that's only $300.
Ah, I see what you mean. OP wrote "300 HUNDRED dollars" (emphasis mine). Slipped right past me. Too hot! :-)Edited: 03 August 2018, 12:09
Even if you manage to smuggle $30k in cash to Japan, you will have difficulty changing into yen without arousing suspicions. Most places put an upper limit, so you will have to go to several places to get it changed. Cash rate isn't particularly good. Best you will get is around 110 yen.
I apologize. I wrote this while I was tired. I just meant 300 dollars.
Question is why you think that small amount would warrant "I don't want to explain why" in your post.
I routinely carry $500 in $100 bills as my emergency stash no matter where I travel. Insignificant in the big scheme of things.....unless there is an emergency.