Monju-no-Yu is the communal bathhouse of Kakeyu Onsen, located in Ueda City, Nagano Prefecture, designated as a National Health Resort Hot Spring by the Ministry of the Environment. The hot spring has an ancient history with a legend that Monju Bosatsu (the Bodhisattva of Wisdom), disguised as a deer, guided a devout hunter to the hot spring location. The spring is believed to have been flowing for approximately 1,200 years, and records show that officials from Kozuke Province visited for therapeutic bathing during the Edo period. The water quality is a simple hot spring (mildly alkaline, low osmotic pressure, high temperature) with a source temperature of 46.0 degrees Celsius, featuring colorless and transparent soft water. In traditional hot spring drinking rankings, Kakeyu Onsen is honored as the Eastern Ozeki (Grand Champion), reflecting its exceptionally palatable and highly regarded drinking water. The facility blends water from five different sources, creating a gentle bathing experience. Monju-no-Yu features both an indoor bath and an open-air bath, effective for hypertension, arteriosclerosis, post-stroke recovery, neuralgia, rheumatism, muscle pain, and fatigue recovery. It serves as the core hot spring of the Maruko Onsen District alongside Reiseniji and Oshio hot springs.