A process for conveniently and efficiently producing a highly pure cinnamaldehyde derivative, for example, (2E)-(3-hydroxy-4-methoxy)cinnamaldehyde, which comprises reacting a benzaldehyde derivative, such as isovanillin and the like, with acetaldehyde in the presence of an alkali, in particular preferably, dropping acetaldehyde little by little thereto in an aqueous solution at a low temperature for the reaction thereof. The cinnamaldehyde derivative thus obtained is selectively reduced to produce 3-(3-hydroxy-4-substituted (with methoxy group or the like)phenyl)propionaldehyde. Further, through a reductive alkylation reaction of the compound thus produced with an aspartame, N-[N-[3-(3-hydroxy-4-substituted (with methoxy group or the like)phenyl)propyl]-L-aspartyl]-L-phenylalanine 1-methyl ester, which is useful as a sweetener having a high sweetening potency, can be industrially and efficiently produced. The processes therefor are also provided.