3-(3-Methyl-4-hydroxyphenyl)-3-methylbutyl aldehyde may be produced from 3-(3-methyl-4-hydroxyphenyl)-3-methylbutyric acid by converting the carboxyl group into a formyl group. N-nullN-null3-(3-methyl-4-hydroxyphenyl)-3-methylbutylnull-L-null-aspartylnull-L-phenylalanine 1-methyl ester, which is useful as a sweetener having a high potency of sweetness, can be easily and efficiently prepared on an industrial production from such an aldehyde derivative, via a reductive alkylation reaction with aspartame. Therefore, 3-(3-Methyl-4-hydroxyphenyl)-3-methylbutyl aldehyde is extremely excellent as an intermediate for the production of N-nullN-null3-(3-methyl-4-hydroxyphenyl)-3- methylbutylnull-L-null-aspartylnull-L-phenylalanine 1-methyl ester. The above butyric acid derivative is a novel compound, which can be easily produced by subjecting 3-(3-methyl-4-methoxyphenyl)-3-methylbutyric acid obtained by the reaction of 2-methoxytoluene with 3-methylcrotonic acid, to a reaction for converting a methoxy group into a hydroxyl group. The present invention further provides several novel compounds useful as a production intermediate, such as the above aldehyde derivative and the like.