The present invention can provide: a prophylactic or ameliorating/therapeutic agent for chronic fatigue syndrome, idiopathic chronic fatigue and fibromyalgia, which contains a processed product of Japanese horseradish (wasabi) as an active ingredient； a food, a beverage, a medicine or a cosmetic, and a medicine for preventing or ameliorating/treating diseases relating to chronic fatigue syndrome, idiopathic chronic fatigue and fibromyalgia, each of which contains the processed product of wasabi； and methods for using these products.
