The present invention provides a quinolone compound represented by General Formula (1) or a salt thereof, wherein R1 represents a hydrogen atom, etc.; R2 represents a hydrogen atom, etc.; R3 represents a phenyl group optionally being substituted with one or more substituents, etc.; R4 represents a halogen atom; R5 represents a hydrogen atom or halogen atom; R6 represents a hydrogen atom; and R7 represents a hydroxyl group, etc. The quinolone compound have a functional improvement effect, which suppresses progression of neurological dysfunction by inhibiting the chronic progression of Parkinson's disease or protecting dopamine neurons from the disease etiology, thereby prolonging the period before first administration begins.