Provided is a single-stranded oligonucleotide that is capable of controlling a target gene with high efficiency and can be easily produced. The single-stranded oligonucleotide is represented by the formula X-L-Y 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 that is capable of hybridizing with a second oligonucleotide and contains at least four contiguous nucleotides recognized by RNase H. Y is composed of 4 to 100 nucleotides, and has a second nucleotide sequence that is capable of hybridizing with a second oligonucleotide and contains at least one ribonucleotide. At least one of nucleotide sequence X and nucleotide sequence Y has an antisense sequence capable of hybridizing with a target RNA. L is a group derived from a third oligonucleotide that is degraded under physiological conditions.