As you know there are many hot springs in Japan called Onsen. There are not only Onsen in hot spring districts, but also in the mountain. If you can take a bath in a hot spring after arrived at your destination in a mountain, you would be so happy.
In fact, there exists such a place in Hakuba area in Northan alps. Hakuba-yari Onsen is located at the point of 2100m on the way to Hakuba-yari-ga-take. It takes about 4 hours hiking from Saru-kura at the foot of Hakuba mountain ranges.
This Onsen is sorounded by the mountains. There is a hut and tent site (1000 yen per person) and is a small brook through tent site that consisting of hot water with hot spring agents.
The temperature of the brook is so comfortable for many people and would heal tired your feet. According to official web site of Hakuba-yari-onsen-goya “白馬鑓温泉小屋: http://www.hakuba-sanso.co.jp/yarionsen/#mark03”, (with / without meal: 9800 yen/6800 yen) the temperature of the hot spring is 43.1 ℃. Other than that , cost of taking a bath in a hot spring is 500 yen per person per night. This means that you can enter the bath any number of times and anytime you want to do it. The most golden time for taking a bath is in early morning, because the terrain of this area is facing towards east direction. So, you would be satisfied with the hot spring bathing in the sun.
[How to access the trailhead]
The closest station to get Hakuba-yari Onsen is Hakuba station on the Itoigawa Line, which runs from Matsumoto to Itoigawa. You need to take a bus from this station. The bus runs from 29th April through to 10th October. The bus fare form Hakuba station is 1000 yen for one way.
Access (Japanese): https://www.alpico.co.jp/access/hakuba/sarukura/
Circuit from Sarukura vaio Daisekkei is the popular plan among hikers. At first, you aproach to at the bottom of Daisekkei for 1 hour walking time from Sarukura. From there it will take 4 hours to get to Hakuba Chojo Syukusya at 2730m. Daisekkei means snow valley, so you need bring crampons.
You can rent it near Hakuba station for 1000 yen. From the hut it will take 2hours Yarigatake 鑓ヶ岳 at 2900m via Shakushidake. Then, go down to Hakuba-yari-onsen-goya via alpine meadow. It takes about 3 hours from the peak of Yarigatake. When you go back to Sarukura, it takes 3 and half hours from Hakuba-yari-onsen-goya.