The present invention can provide a nucleic acid medicine which has a higher effect and a more prolonged effect of inhibiting the expression of α-synudein can be provided. Disclosed is the oligonucleotide or a pharmacologically acceptable salt thereof, the oligonucleotide containing at least one nucleoside structure represented by Formula (I): (where each of Base and A are defined substituent or structure), can bind to an α-synudein gene, has activity for inhibiting expression of the α-synudein gene, and is complementary to the α-synudein gene, and the oligonucleotide has a length of twelve to twenty bases.