The purpose of the present invention is to provide a chemically-modified siRNA in which polydeoxyadenylic acid is added to the 5' end of the sense strand thereof, wherein when the chemically-modifiedsiRNA is complexed with sizofiran, the chemically-modified siRNA becomes highly resistant to RNase, and effectively exhibits RNAi activity. The problem can be solved by, in a modified siRNA in which polydeoxyadenylic acid is added to the 5' end of the sense strand thereof, applying a specific chemical modification to dinucleotide sequences CA, UA, and UG in the base sequence of the sense strand, and to dinucleotide sequences CA, UA, and UG located at or beyond the eighth base from the 5' end in the base sequence of the antisense strand.