Described is a process for producing a 4-substituted-2-butenal represented by the following formula (1); ##STR1## (wherein X represents an acyloxy group or halogen atom; R represents hydrogen atom, an aliphatic hydrocarbon group or an aromatic hydrocarbon group; and these hydrocarbon groups can be substituted with hydroxyl group, an alkoxy group, an aryloxy group, an acyl group or an alkoxycarbonyl group), which is useful as synthetic intermediates of phormaceutieals, agricultural chemicals and the like, comprising subjecting a substituted acetaldehyde represented by the following formula (2); X--CH.sub.2 --CHO (2) (wherein X has the same meaning as defined above), with an aldehyde represented by the following formula (3); R--CH.sub.2 --CHO (3) (wherein R has the same meaning as defined above) in the presence of an amino carboxylic acid.