Karurusu Onsen is a historic hot spring resort located in Noboribetsu City, Hokkaido. Discovered in 1886 and opened in 1899, it was named after Karlsbad (Karlovy Vary) in the Czech Republic due to its similar water quality. In 1957, it became the first National Health Resort Hot Spring in Hokkaido and the 12th in Japan, and is also selected as one of Japan's 100 Famous Hot Springs. Located about 8km northwest of Noboribetsu Onsen, this quiet spa town sits along the Noboribetsu River at the foot of Mt. Kita-uma and Mt. Orofure. The water is classified as simple thermal spring (low-tension neutral high-temperature spring), nearly colorless and odorless, but contains sulfate spring components known as one of the three major skin-beautifying spring types. Rich in metasilicic acid, it offers excellent moisturizing and blood circulation benefits. Source temperature ranges from 48-68 degrees Celsius, and all ryokan offer free-flowing natural hot spring water. In contrast to the bustling Noboribetsu Onsen, Karurusu offers a peaceful therapeutic atmosphere befitting its reputation as the inner retreat.