Large-scale SNP analyses conducted on subjects in a Parkinson's disease patient group and a normal control group led to the successful identification of a gene (GRK5) associated with Parkinson's disease. In addition, it was newly discovered that phosphorylation of .alpha.-synuclein is promoted by enhanced expression of the GRK5 gene, and as a result, the formation of soluble .alpha.-synuclein oligomers is promoted, leading to Parkinson's disease. The present invention enables the assessment of Parkinson's disease as well as the screening of therapeutic agents for Parkinson's disease using as an index the expression of GRK5 gene.