The invention discloses an oligonucleotide molecule used for inhibiting expression of mRNA of a target gene and a composition set thereof, and provides siRNA which is composed a positive-sense strand which is composed of 19-27 nucleotides and a negative-sense strand being reverse-complementary with the positive-sense strand. In the positive-sense strand, both 5-9 continuous nucleotides from a 5'-terminal and 5-9 continuous nucleotides from a 3'-terminal are 2'-ribose modified nucleotides. The siRNA molecular mixture can influence expression of target genes in at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% and 90% cells, and is at least 45%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90% and 95% in inhibition ratio.