I will explain the tax at the time I made a profit in Las Vegas casino.
There is a tax agreement in Japan and the United States. It is a convention concerning the treatment of tax over the country.
People living in Japan are obliged to pay taxes in Japan, so it is not necessary to pay tax in the United States. Income in Las Vegas will be taxed 30%.
This 30% is money to pay in Japan, you do not have to pay in the US. Depending on the actual casino, there seems to be some deduction of 30%. In the case of deducting 30% tax, since it is a tax which does not need to be paid originally as mentioned above, you can get back to the US government at a later date by refunding procedure.
If you have a number called ATIN (ITIN) number you do not need to pay taxes to the US, but if you win in Las Vegas and you want to return home you will need to receive a certificate issued by the casino. This will be a source certificate of cash when leaving America. This will be sent to Japanese tax authorities.