If you get lost during the climb, it will cost an enormous amount to look for you (Particularly, in the case of using helicopters or taking a long time for searching). Purchasing insurance in advance would be safer.
The Japan Tourism Agency encourages foreign visitors to purchase traveler’s insurance, but it does not usually cover sports that include dangerous risks such as mountain climbing. Please purchase (mountaineering) insurance cover in your country beforehand.

• Japan Tourism Agency (Check "Travel Insurance" tag)