Spider fighting, a seasonal natural sport historically popular in many parts of Japan, has declined or disappeared in many areas because of changes in the social and natural environment during post-World War II economic growth. However, in some areas of Kanto, Kinki, Shikoku, and Kyushu, spider fighting continues to be practiced as a regional event. This study elucidated the structure of participants’ folk knowledge using the case of a spider-fighting event in the town of Kajiki, Kagoshima prefecture. The relationship between the sustainable use of spiders and folk knowledge was also examined. The participants gained knowledge through four stages of activities: collecting; raising; fighting; and releasing spiders. Obtaining and utilizing spiders requires diverse knowledge. The participants’ perspectives extended to other animals and plants that support the survival of spiders. The knowledge at each stage was not considered to be independent but rather to have a dialectical structure that interacts and changes with other stages. Amid the changing environment, the participants continue to develop wisdom for living with spiders, which might be viewed as the significance of the spider-fighting event.