Juvenile fibromyalgia (JFM) is often diagnosed at a later stage. Therefore, little is known about its early phase. A 12-year-old girl with persistent lumbago without fever consulted an orthopedist, but imaging studies showed no abnormalities and analgesics were ineffective. She therefore visited our pediatric clinic. On digital palpation, she had pain in 13 of the 18 tender point sites of fibromyalgia. The blood test results were unremarkable. A medical interview revealed character tendencies often seen in patients with JFM;she was serious-minded, uncompromising, and showed excessive concern for others. Furthermore, psychological stress for the approaching annual sports day might have built up. All factors considered, early-stage JFM was the probable diagnosis. The patient accepted the diagnosis and understood that the disease may be psychogenic. Getting through the sports day cured her symptoms. This case highlights the importance of initial care, including a detailed explanation, in those with JFM.