Tel: 0558-35-7711
Address: 359 Nashimoto Kawazu-cho , Kamo-gun Shizuoka Japan, 413-0501
How to get there: From Tokyo station take the Tokyu (express) Odori-ko to Kuwazu station. 
Official site

Located between Shimoda and Shuzenji, Amagisou onsen has 12 baths (7 inside and 5 out). There are two mixed cave baths and more segregated baths are located in an around the main building of the ryokan. 

The onsen has a large number of baths and also a big pool. The surroundings are pleasant and alot of the baths are next to a river, making for a great view with no buildings in site.