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.提供高效率地控制標的基因，且可容易地製造之單股寡核苷酸。一種單股寡核苷酸，其係以式(I)表示，X係以Xa-Xb表示，Xa連結於Y，Xb與Y雜交。Xa係由1~40個核苷酸所構成，且至少包含1個修飾核苷酸。Xb係由4~40個核苷酸所構成，且至少包含1個修飾核苷酸。Y係由4~40個核苷酸所構成，且至少包含1個核糖核苷酸。Xz、Yz係由5~40個核苷酸所構成，且至少包含1個修飾核苷酸。核苷酸序列X、Xz、Yz，具有可與標的RNA雜交之反義序列。Lx及Ly係由0~20個核苷酸所構成。
