The invention provides a single-stranded oligonucleotide represented by the formula [Xz-Lx]m-X-Y-[Ly-Yz]n, wherein X is represented by Xa-Xb, Xa is coupled with Y, and Xb and Y hybridize. Xa is composed of 1 to 40 nucleotides and contains at least one modified-nucleotide. Xb is composed of 4 to 40 nucleotides and contains at least one modified-nucleotide. Y is composed of 4 to 40 nucleotides and contains at least one ribonucleotide. Xz and Yz are composed of 5 to 40 nucleotides and contain at least one modified-nucleotide. Nucleotide sequences X, Xz and Yz have an antisense sequence capable of hybridizing with a target RNA. Lx and Ly are composed of 0 to 20 nucleotides.