Kawazoko Onsen Hotaruseiso is a historic secluded one-inn hot spring located in Sugawara, Kokonoe-machi, Kusu-gun, Oita Prefecture. The hot spring is said to have been discovered in August of the first year of Engi (901 AD) when Sugawara no Michizane was hiding at Hakuunzan Jomei-ji temple in Sugawara village to escape assassins while being exiled to Dazaifu. The place name "Sugawara" is also derived from Sugawara no Michizane. In the 3rd year of Ansei (1856), the village headman Sugawara Gentoku developed the hot spring for the villagers, creating the current stone-paved bathing pools and building a bathhouse inn.

The greatest feature of this hot spring is its natural floor-spring source that has been flowing for over 1,100 years. The facility is built along the Machida River, with three beautiful stone bathing pools arranged at the same level as the river surface. These pools were constructed during the Edo period (1856) and have approximately 170 years of history. The bottom of the pools is paved with round stones, and hot spring water emerges from the floor. Each pool has a different temperature, with the innermost pool being the hottest, the middle one moderately hot, and the front one lukewarm, creating a graduated temperature structure.

The colorless, transparent, and extremely clear water is a simple hot spring (alkaline simple thermal spring) with a gentle quality that can be safely enjoyed by everyone from babies to elderly people. The facility suffered flood damage in the July 2020 heavy rains and was temporarily closed, but reopened around 2021 with newly reconstructed buildings. However, the historic stone-paved bathing pools in the main bath have been preserved as they were, allowing visitors to experience the historical atmosphere. In April 2024, a sauna and outdoor bath were added, enhancing the facilities. True to its name "Hotaruseiso" (Firefly River Lodge), the location is surrounded by rich nature where fireflies can be observed, and guests can enjoy the hot spring while listening to the sound of the flowing Machida River.