Provided is a novel nucleic acid molecule that can inhibit the expression of a gene and can be produces easily and efficiently. The nucleic acid molecule is a single-stranded nucleic acid molecule including an expression inhibitory sequence that inhibits expression of a target gene. The single-stranded nucleic acid molecule includes, in sequence from the 5' side to the 3' side: a 5' side region (Xc)； an inner region (Z)； and a 3' side region (Yc). The inner region (Z) is composed of an inner 5' side region (X) and an inner 3' side region (Y) that are linked to each other. The 5' side region (Xc) is complementary to the inner 5' side region (X). The 3' side region (Yc) is complementary to the inner 3' side region (Y). At least one of the inner region (Z), the 5' side region (Xc), and the 3' side region (Yc) includes the expression inhibitory sequence. According to this single-stranded nucleic acid molecule, it is possible to inhibit the expression of the target gene.
