A method of producing 1,2,4-triazole, comprising a step of subjecting a ketazine, water and formamide to a cyclization reaction. The ketazine and water are added simultaneously to formamide kept at a cyclization temperature. Then, ketone by-produced during the cyclization reaction is distilled off while maintaining the reaction mixture at a cyclization temperature to complete the cyclization reaction, thereby obtaining 1,2,4-triazole. The use of the ketazine in place of hydrazine makes the energy-consuming hydrolysis step in the conventional hydrazine production unnecessary. Therefore, the method of the invention is advantageous over the known methods in energy consumption and production cost.