The invention provides a single-stranded nucleic acid capable of inhibiting expression of a target gene having a delivery function. The nucleic acid contains, from the 5′-side to the 3′-side, a 5′-side region (Xc), a linker region (Lx), an inner region (Z), a linker region (Ly) and a 3′-side region (Yc) in this order, wherein the inner region (Z) is constituted by linkage of the inner 5′-side region (X) and the inner 3′-side region (Y), 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) comprises an expression inhibitory sequence that inhibits expression of a target gene, and at least one of the 5′-terminus, the 3′-terminus, the linker region (Lx) and the linker region (Ly) is bound to a bio-related substance.