In the production of an alkylbenzaldehyde by a method comprising a step of preparing a solution of complex of a starting alkylbenzene and a hydrogen fluoride-boron trifluoride catalyst and a step of formylation by bringing the solution of complex into contact with carbon monoxide, an alkylbenzene having at least one primary alkyl group having two or more carbon atoms on its benzene ring is used as the starting alkylbenzene. The preparation of the solution of complex is carried out in the presence of an aliphatic or alicyclic saturated hydrocarbon having 6 to 10 carbon atoms which contains at least one tertiary carbon atom but contains no quaternary carbon atom. By the combined use of the specific alkylbenzene and the aliphatic or alicyclic saturated hydrocarbon, the disproportionation of the alkylbenzene is prevented and the alkylbenzaldehyde is produced at high yields.