Method for producing heteroaromatic aldehydes

A process for preparing a heteroaromatic aldehyde by catalytic reaction of an alkyl-substituted heteroaromatic compound with molecular oxygen in a gaseous phase in the presence of a catalyst, which comprises employing an oxide containing vanadium, phosphorus, aluminum and silicon as a catalyst, and diluting a part or all of the oxide present as a catalyst layer in a reactor with a solid inert to the reaction, wherein the alkyl-substituted heteroaromatic compound is at least one selected from the group consisting of an alkyl-substituted pyridine, an alkyl-substituted pyrazine and an alkyl-substituted pyrimidine.