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 gene's mRNA, 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.