Provided is a single-stranded oligonucleotide capable of controlling a gene of interest at high efficiency and being produced easily. A single-stranded oligonucleotide represented by formula (I), where X is represented by Xa-Xb, Xa links to Y, and Xb and Y hybridize. Xa comprises 1-40 nucleotides and includes at least one modified nucleotide. Xb comprises 4-40 nucleotides and includes at least one modified nucleotide. Y comprises 4-40 nucleotides and includes at least one ribonucleotide. Xz and Yz comprise 5-40 nucleotides and include at least one modified nucleotide. Nucleotide sequences X, Xz, and Yz have an antisense sequence that can hybridize with an RNA of interest. Lx and Ly comprise 0-20 nucleotides.
