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.