A visit to Shibu Onsen is a historical and unique experience. Shibu is a hot springs/onsen town in the Nagano Prefecture that dates back 1,300 years. The Master Key refers to a key that gets you entry to nine Bath House's called Sotoyu Public Baths. In order to get the Master Key, you must stay with an accommodation that holds the Master Key. Some Ryokan, Inn or Hotel's pay for the upkeep of the Sotoyu Public Baths; like an Association. Finding the places that hold the Master Key can take a lot of research. I'm offering up the name of the Ryokan I stayed at to save you some research: Sakaeya Ryokan not only held the Master Key, it was my favorite accommodation on my trip to Japan.
First off you will need plenty of time to visit the nine Onsen as it might take some time to cool off particular baths, some of them were impossibly hot, other were already perfect. Our Ryokan gave use a detailed Shibu Onsen map; each onsen is good for specific ailments; stomach, intestinal, skin, acne, gout, eye, injuries, rheumatism and women's health.
Be prepared to walk in wooden shoes and a Yukata (an informal summer kimono) down many streets to get to all nine Onsen.
You can also get a special cloth that you stamp with a commemorative seal at each onsen, after stamping all nine onsens and one at the Takayakushi Shrine, our Ryokan stamped it with the date! If you want to skip the overnight stay then you can stop at the Shibu Onsen Information Center and purchase a ticket for 500 Yen to one of the nine Onsen called Oy-u (number nine on the map). You will then go to a designated local Inn or Shop to get the key. Keep in mind that the famous Jagokundani Snow Monkey Park is very close to Shibu, although I visited offseason and it didn't seem busy it's possible it might be crowded in the winter.