The invention provides a single-stranded oligonucleotide represented by the formula (I), wherein X and Y hybridize by a first nucleotide sequence portion and a second nucleotide sequence portion. X is composed of 7 to 100 nucleotides, contains at least one modified-nucleotide, and has a first nucleotide sequence capable of hybridizing with a second oligonucleotide. Y is composed of 4 to 100 nucleotides, enables hybridization with the above-mentioned first oligonucleotide, and has a second nucleotide sequence containing at least one ribonucleotide. At least one of the nucleotide sequences X, Xz and Y has an antisense sequence capable of hybridizing with a target RNA. At least one of L, Lx and Ly is a linking group that contains a non-nucleotide structure.