An object of the present invention is to provide a method for producing m-dialkylbenzaldehyde by using a reaction starting material containing 1,4-dialkylbenzene. The method for producing m-dialkylbenzaldehyde represented by formula (3), comprising a step of allowing carbon monoxide to react on a reaction starting material containing 1,4-dialkylbenzene represented by formula (1) in the presence of a Bronsted acid and a Lewis acid, wherein the reaction starting material is 1,4-dialkylbenzene represented by formula (1), or a mixture of 1,4-dialkylbenzene represented by formula (1) and 1,3-dialkylbenzene represented by formula (2), containing 10 mol % or more of the 1,4-dialkylbenzene represented by formula (1),
wherein in formulae (1) to (3), R1 represents a methyl group or an ethyl group, and R2 represents a chain or cyclic alkyl group having 3 or more and 6 or less carbon atoms that has a tertiary carbon at the benzyl position.