Both Glenski and JerseyBoy are right about the NHI and the salaryman's Shakai Hoken covering the medical cost in certain cases outside Japan.
But the GaijinPunch's case, the 1st one, is about his trip for the treatment purpose.
I'm wondering what would happen if I got a chronic illness and wanted to move back to the US.
I don't think this case will be covered.
Even travel insurance does not cover treatment of your chronic disease except some special ones for expats and their family going to another countries.
*snip
Some life insurance policies cover the hospitalization outside Japan.
*
I got a wakeup call of sorts when I went to the US last month and had to visit the ER.
In this case, you may have right to claim the refund like I did.
You need to translate some documents such as...
Attending Physician's Statement
Receipt for your treatment and drug prescription
The best case is to bring the designated forms anytime you go outside Japan.
But in my case, I didn't bring any documents to the country, actually I didn't know the system, but the doctor kindly arranged the documents in English. Then I translated them in accord with the insurance policy as instructed by the city office staff and submitted them to the municipal office.
The 70% refund is calcurated on the standard medical practices in Japan.
So at inexpensive treatment countries, you can expect similar refund like in Japan, but in the US, the best treatment as well as expensive... I don't know.
I don't know how much you paid and will be refunded for the immediate case, but at first it is good to go to your municipal office for
kaigai ryoyo hi system, ナ?Cナ?O窶氾??倍窶敕ッツ青ァ窶忸.