Biomarker, method for discriminating parkinson's disease from parkinson's syndrome, method for predicting onset risk of parkinson's disease and method for evaluating beneficial effect of drug on parkinson's disease

[Problem] To provide: a biomarker that is useful for quickly, safely, noninvasively, easily and surely discriminating Parkinson's disease from Parkinson's syndrome while reducing a burden on a subject or individual, predicting the onset risk of Parkinson's disease, or evaluating a beneficial effect of a drug on Parkinson's disease; a method for quickly, safely, noninvasively, easily and surely discriminating Parkinson's disease from Parkinson's syndrome while reducing a burden on a subject or individual; a method for predicting the onset risk of Parkinson's disease; and a method for evaluating a beneficial effect of a drug on Parkinson's disease. [Solution] A biomarker, which comprises isatin, for discriminating Parkinson's disease from Parkinson's syndrome, for predicting the onset risk of Parkinson's disease and/or for evaluating a beneficial effect of a drug on Parkinson's disease.