The present invention relates to oligo double-stranded RNAs for use in knockdown of the expression of Bcl-2 protein known as a suppressor for apoptosis, and pharmaceutical compositions containing them. It is known that Bcl-2 protein is over-expressed in diseases such as cancer, and by this over-expression, the growth of cancer cells is continued and the drug resistance to anti-cancer agents is caused. The present invention provides highly active oligo double-stranded RNAs cleaving bcl-2 mRNA and further provides pharmaceutical compositions comprising a complex of the oligo double-stranded RNA and a suitable carrier for inhibiting the expression of Bcl-2 protein.