The present invention provides a naturally occurring miRNA characterized in that: the naturally occurring miRNA is a single-stranded nucleic acid having an X region and a Y region； the 3' end of the X region and the 5' end of the Y region link via a linker region of a non-nucleotide structure； the X region contains (a) a guide strand sequence or (b) a passenger strand sequence of a mature miRNA； in case of (a), the Y region contains a passenger strand sequence of this mature miRNA, whereas in case of (b), the Y region contains a guide strand sequence of this mature miRNA； and the guide strand sequence and passenger strand sequence form a double-stranded structure.
