This park is located at the foot of the Shiga Kogen snow resort, and it is known for its wild snow monkeys which come down from the forests and mountains to immerse themselves in the hot spring bath. They especially like to do so during the cold winter season.
We arrived at the entrance to the park, and then began a 25-30 minute walk (about 1.6 km upslope) to the hot spring. It was a bit difficult to walk on the slippery road, due to the snow, but luckily we had borrowed snow boots from the ryokan where we stayed at.
We had done a bit of research before the trip, and judging from the information given by the live cam at the park, most of the monkeys would come down to the hot spring after 11 am. We reached the park at about 2.30 pm, and there were lots of monkeys present during that time.
Some things to note: Visitors should not feed the monkeys or touch them. Pets are also not allowed in the park. The monkeys are very used to having their photos taken, so using a flash camera is not a problem.
The entrance fee is 500 Yen per adult. The park closes at 4 pm during the winter season.
Without a doubt, one of the coolest spectacles in all of Japan is to watch the wild snow monkeys of the Japanese Alps sit out a snowy winter day soaking in the hot waters of a naturally-heated onsen. At Jigokudani Yaenkoen monkey park, just above Nagano, you can see these migratory macaques, the world's most northerly primates (besides man) forage for food under the snow and soak in an onsen to relax after a hard days work. The monkeys can be reached after a 1.6 km hike over a wide path covered in cedars along the Yogoku River (itself a beautiful activity during a snowfall). At the end, you'll reach a small cabin where you'll pay Y500 to go see an onsen that was originally meant for people but taken over by the monkeys. There were around 20 of them bathing at once when I visited during a somewhat heavy snowfall. It's very interesting to watch them interact or sleep, much as humans would, as they take to the waters. Of course, they are very active and scurrying around accomplishing tasks other than bathing -- during which they seem to pay you no heed. I wouldn't touch them or harass them -- wild monkeys can hurt you -- but you can get pretty close without them seeming to mind.
....Jigokudani literally means "Hell's Valley" and is located 850 meters above sea level, so snow covers the area about a third of the year. I have been told that some macaques hang out there year round, but are most easily seen in the winter when they congregate around the onsen.
Is a little out of the way : 40 mins by local train to Yudanka Station, 15 min by bus to the Kayanbashi than a 20 min hike to the park