PURPOSE: To obtain the aimed compound in high space time yield and industrially advantageously in producing formaldehyde by partially oxidizing methane with oxygen or oxygen-containing gas, by using a compound containing molybdenum and bismuth as a catalyst.

CONSTITUTION: In producing formaldehyde useful as a raw material for phenolic type, urea type or melamine type synthetic resins, etc., by partially oxidizing methane with oxygen or oxygen-containing gas, a compound containing molybdenum and bismuth as essential components is used as a catalyst and the reaction is carried out while preferably supplying a feed gas with 0.1W35vol%, especially 1W20vol% steam to give the aimed compound by direct oxidation of methane by one stage and in high space time yield. Selectivity of the aimed compound is improved by steam supply. Further the catalyst may be mixed with another element such as alkali metal, etc., in order to improve physical properties and performances of the catalyst.