A method produces a hairpin single-stranded RNA molecule capable of inhibiting expression of a target gene, including the step of reacting a first single-stranded oligo-RNA molecule represented by formula (I) with a second single-stranded oligo-RNA molecule represented by formula (II) in a mixed solvent including a buffer solution and a hydrophilic organic solvent in the presence of a dehydration condensation agent: 5′-Xc-Lx1 . . . (I) and Lx2-X—Y-Ly-Yc-3′ . . . (II), wherein the dehydration condensation agent is selected from the group consisting of a triazine-based dehydration condensation agent, a uronium-based dehydration condensation agent including an N-hydroxy nitrogen-containing aromatic ring structure, a carbodiimide-based dehydration condensation agent, a 2-halopyridinium-based dehydration condensation agent, and a formamidinium-based dehydration condensation agent.