A single-stranded RNA molecule comprising: only one expression inhibitory sequence that inhibits expression of a target gene, wherein the 5' end and the 3' end are not linked to each other, the single-stranded RNA molecule comprises, in sequence from a 5' side to a 3' side: a 5' side region (Xc); an inner region which comprises the expression inhibitory sequence; and a 3' side region (Yc), the inner region is composed of an inner 5' side region and an inner 3' side region that are linked to each other, the 5' side region (Xc) is complementary to the inner 5' side region and a double strand is formed by fold-back of the region (Xc) towards the region and self annealing of the regions (Xc) and, the 3' side region (Yc) is complementary to the inner 3' side region and a double strand is formed by fold-back of the region (Yc) towards the region and self annealing of the regions (Yc) and, a linker region (Lx) is present between the 5' side region (Xc) and the inner 5' side region, the 5' side region (Xc) and the inner 5' side region are linked to each other via the linker region (Lx), the linker region (Lx) comprises a non-nucleotide residue, a linker region (Ly) is present between the 3' side region (Yc) and the inner 3' side region, the 3' side region (Yc) and the inner 3' side region are linked to each other via the linker region (Ly), and the number of bases in the inner region, the number of bases in the inner 5' side region, the number of bases in the inner 3' side region, the number of bases (Xc) in the 5' side region (Xc), and the number of bases (Yc) in the 3' side region (Yc) satisfy conditions (a) to (d): (a) Conditions of Expressions and are satisfied: Z = X + Y Z ≥ Xc + Yc (b) Z = 19 to 30; (c) Xc = 1 to 7 or Yc = 1 to 9; and (d) Z - (Xc + Yc) = 0 or 1 to 10. only one expression inhibitory sequence that inhibits expression of a target gene, wherein the 5' end and the 3' end are not linked to each other, the single-stranded RNA molecule comprises, in sequence from a 5' side to a 3' side: a 5' side region (Xc); an inner region which comprises the expression inhibitory sequence; and a 3' side region (Yc), the inner region is composed of an inner 5' side region and an inner 3' side region that are linked to each other, the 5' side region (Xc) is complementary to the inner 5' side region and a double strand is formed by fold-back of the region (Xc) towards the region and self annealing of the regions (Xc) and, the 3' side region (Yc) is complementary to the inner 3' side region and a double strand is formed by fold-back of the region (Yc) towards the region and self annealing of the regions (Yc) and, a linker region (Lx) is present between the 5' side region (Xc) and the inner 5' side region, the 5' side region (Xc) and the inner 5' side region are linked to each other via the linker region (Lx), the linker region (Lx) comprises a non-nucleotide residue, a linker region (Ly) is present between the 3' side region (Yc) and the inner 3' side region, the 3' side region (Yc) and the inner 3' side region are linked to each other via the linker region (Ly), and the number of bases in the inner region, the number of bases in the inner 5' side region, the number of bases in the inner 3' side region, the number of bases (Xc) in the 5' side region (Xc), and the number of bases (Yc) in the 3' side region (Yc) satisfy conditions (a) to (d): (a) Conditions of Expressions and are satisfied: Z = X + Y Z ≥ Xc + Yc (b) Z = 19 to 30; (c) Xc = 1 to 7 or Yc = 1 to 9; and (d) Z - (Xc + Yc) = 0 or 1 to 10. (a) Conditions of Expressions and are satisfied: Z = X + Y Z ≥ Xc + Yc (b) Z = 19 to 30; (c) Xc = 1 to 7 or Yc = 1 to 9; and (d) Z - (Xc + Yc) = 0 or 1 to 10.