A double-stranded polynucleotide or a salt thereof, consisting of a polynucleotide represented by the formula (I) and a polynucleotide represented by the formula (II) and having the following characteristics (a) to (d): 5 - X - (α - β) q - αp - λm - 3 (I) and 5 - δs - (α - β) r - γ - Vn - 3 (II), (a) α and β represent differently one DNA or a 2-OMeARN, δ and λ represent identically or differently a DNA or a 2-OMeARN, δ and λ represents identically or differently any nucleotide selected from a DNA, an RNA, and a 2-OMeARN and X and Y represent an oligonucleotide consisting of the same or different kinds of nucleotides selected from a DNA, an RNA and a modified nucleic acid (B) p represents an integer of 0 or 1, m is 0 when p is 0 and represents any integer from 0 to 5 when p is 1, q represents any integer from 3 to 9, the total number of nucleotides in (α-β) q and X es17 or 18, s represents an integer of 0 or 1, n represents any integer of 0 to 5, r represents any integer from 3 to 9 and the total number of nucleotides in (α- β) re Y is 17 or 18 (C) X- (α-β) q-αp in the polynucleotide represented by the formula (I) has a nucleotide sequence identical to a target gene and (d) X- (α-β) q in formula (I) and (α-β) rY in formula (II) have nucleotide sequences complementary to each other.Un polinucleótido bicatenario o una sal del mismo, que consiste en un polinucleótido representado por la fórmula (I) y un polinucleótido representado por la fórmula (II) y que tiene las siguientes características (a) a (d): 5 - X - (α - β)q - αp - λm - 3 (I) y 5 - δs - (α - β)r - γ - Vn - 3 (II) , (a) α y β representan de forma diferente un ADN o un 2-OMeARN, δ y λ representan de forma idéntica o diferente un ADN o un 2-OMeARN, δ y λ representa de forma idéntica o diferente cualquier nucleótido seleccionado de un ADN, un ARN y un 2-OMeARN, y X e Y representan un oligonucleótido que consiste en los mismos o diferentes tipos de nucleótidos seleccionados de un ADN, un ARN y un ácido nucleico m
