The present invention provides a composition that comprises a lipidparticle encapsulating a double-stranded nucleic acid molecule, wherein the antisense strand is a polynucleotide of 17 to 30 bases in which a sequence is complementary to the sequence of the 17 contiguous bases of a target genes RNA, the sense strand is a polynucleotide of 17 to 30 bases that contains a base sequence complementary to the base sequence of bases 1 to 17 in the 5-end to 3-end direction of the antisense strand, and a particular amount of the sugars binding to certain bases of the antisense strand and the sense strand are deoxyribose, or ribose whose hydroxyl group at the 2 position is substituted by a modifying group, and wherein the lipidparticle contains a lipid bilayer membrane whose constituent component is a lipid conjugate, a fatty acid conjugate or an aliphatic hydrocarbon conjugate of a water-soluble substance.
