A method for producing a hairpin single-stranded RNA molecule capable of inhibiting expression of a target gene, comprising the step of reacting a first single-stranded oligo-RNA molecule represented by the following formula (I) with a second single-stranded oligo-RNA molecule represented by the following formula (II) in a mixed solvent comprising a buffer solution and a hydrophilic organic solvent in the presence of a dehydration condensation agent: 5'-Xc-Lx 1 ... (I) Lx 2 -X-Y-Ly-Yc-3' ... (II) , where in the formula (I) or (II), X, Xc, Y and Yc each is composed of a ribonucleotide residue, Xc is complementary to X, Yc is complementary to Y, Ly is a non-nucleotide linker, Lx 1 is a non-nucleotide linker having an amino group, Lx 2 is a non-nucleotide linker having a carboxyl group, and X-Y comprises a gene expression-inhibiting sequence for the target gene, wherein the dehydration condensation agent is selected from the group consisting of a triazine-based dehydration condensation agent, a uronium-based dehydration condensation agent comprising 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, wherein if the dehydration condensation agent is the carbodiimide-based dehydration condensation agent, the dehydration condensation agent is used in combination with an N-hydroxy nitrogen-containing aromatic compound or a cyano(hydroxyimino)acetic acid ester, wherein if the dehydration condensation agent is the 2-halopyridinium-based dehydration condensation agent, the dehydration condensation agent is used in combination with an N-hydroxy nitrogen-containing aromatic compound, and wherein if the dehydration condensation agent is the formamidinium-based dehydration condensation agent, the dehydration condensation agent is used in combination with an N-hydroxy nitrogen-containing aromatic compound or an N-hydrocarbon-substituted imidazole derivative. where in the formula (I) or (II), X, Xc, Y and Yc each is composed of a ribonucleotide residue, Xc is complementary to X, Yc is complementary to Y, Ly is a non-nucleotide linker, Lx 1 is a non-nucleotide linker having an amino group, Lx 2 is a non-nucleotide linker having a carboxyl group, and X-Y comprises a gene expression-inhibiting sequence for the target gene, wherein the dehydration condensation agent is selected from the group consisting of a triazine-based dehydration condensation agent, a uronium-based dehydration condensation agent comprising 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, wherein if the dehydration condensation agent is the carbodiimide-based dehydration condensation agent, the dehydration condensation agent is used in combination with an N-hydroxy nitrogen-containing aromatic compound or a cyano(hydroxyimino)acetic acid ester, wherein if the dehydration condensation agent is the 2-halopyridinium-based dehydration condensation agent, the dehydration condensation agent is used in combination with an N-hydroxy nitrogen-containing aromatic compound, and wherein if the dehydration condensation agent is the formamidinium-based dehydration condensation agent, the dehydration condensation agent is used in combination with an N-hydroxy nitrogen-containing aromatic compound or an N-hydrocarbon-substituted imidazole derivative. <EOS>