An agent for preventing or treating a functional brain disease and/or inhibiting symptom development of the functional brain disease includes (2R)-2-propyloctanoic acid or a salt thereof. The agent may be safely administered to patients of, for example, functional brain diseases such as depression, menopausal mood disorder, perimenopausal mood disorder, panic disorder, irritable bowel syndrome, social anxiety disorder, post-traumatic stress disorder or the like, and also can show excellent effects.
