It's called Ninja Beer because it's a combination of hot springs water and beer and is being used for spa treatments. The two world's meet as there is a brewery not too far from the resort.
For about R212 ($15) you can enjoy a 45-minute session in a barrel-like tub at the Hinotani Onsen Misugi Resort.
The Asahia Shimbun reports that the beer tap and the water tap runs simultaneously and that hotel staff encourages guests to drink from the beer spa if they like the temperature of the brew/spa water.
The news outlet also reports that: "The hotel installed the 1.2 meter-diameter tub along with an actual beer tank and server recently when renewing its communal bath. The beer is diluted more than 200 times, making the alcohol less than 0.03 percent of the liquid."
According to the resort's website, it is built on the site of a working brewery which produces over 30 000 litres of beer annually.
There are no known health benefits to beer baths, but clients at the resort and staff claim that beer opens pores and clears skin of impurities.