The present invention provides a pharmaceutical agent that inhibits the chronic progression of Parkinson's disease or protects dopamine neurons from disease etiology, thereby suppressing the progression of neurological dysfunction, so as to prolong the period of time until L-dopa is administered while also improving neuronal function; the pharmaceutical agent of the invention comprising as an active ingredient a quinolone compound represented by Formula (1): or a salt thereof, wherein: R1 represents hydrogen or the like; R2 represents hydrogen or the like; R3 represents substituted or unsubstituted phenyl or the like; R4 represents hydrogen or the like; R5 represents hydrogen or the like; R6 represents hydrogen or the like; and R7 represents hydroxy or the like.