The objective of the present invention is to provide nucleic acid therapeutics which exhibits more excellent effect and which shows a substantivity for a prolonged period to suppress an expression of α-synuclein. The oligonucleotide or a pharmacologically acceptable salt thereof according to the present invention is characterized in comprising at least one 2′-O,4′-C-ethylene nucleoside, wherein the oligonucleotide can hybridize with α-synuclein gene, has an activity to suppress an expression of the α-synuclein gene, and is complementary to the α-synuclein gene, 5′ end of the oligonucleotide is a nucleotide complementary to the specific nucleotide, the oligonucleotide is complementary to at least a part of SEQ ID NO: 1, and the oligonucleotide has a length of 13 or more and 15 or less nucleotides.