To provide a method for treating drainage containing hexamine which can treat the drainage containing hexamine securely and also more economically, and be applied to practical drainage treatment.

The method for treating the drainage containing hexamine has the process of filling the drainage into a pressure vessel, and that of decomposing hexamine under conditions of temperatures in the vessel of 200 to 250°C and pressures in the vessel of 1.5 to 2.5MPa.