PURPOSE: To prepare the titled compound in high yield, by the catalytic oxidation reaction of methane with an oxidizing agent capable of easily generating oxygen anion radical, in the presence of an oxidation catalyst comprising an oxide of tungsten, etc.

CONSTITUTION: The titled compound can be prepared by reacting methane with an oxidizing agent capable of easily generating oxygen anion radical, e.g. dinitrogen monoxide, nitrogen monoxide, etc. in the presence of an oxidation catalyst comprising one or more kinds of vanadium, chromium and tungsten oxides, preferably in the presence of steam, at 450W650°C, preferably 500W600°C. The catalyst can be prepared by supporting the oxide of vanadium, chromium or tungsten on a carrier, preferably silica gel, by impregnation, etc., drying the mixture with heat, and calcining in air. the amount of the oxidizing agent is preferably 0.5W10mol, especially 0.05W5mol per 1mol of methane.