In the separation and purification of N-[N-(3,3-dimethylbutyl)-L-&agr;-aspartyl]-L-phenylalanine methyl ester produced in the reductive alkylation of aspartame (APM) with 3,3-dimethylbutylaldehyde, the APM used in the starting material for the alkylation, which is difficult to be separated by recrystallization only from the crude crystals of the reaction product, can be separated and removed by extraction or solution thereof with the specified organic solvent from the concentrated reaction solution of the reductive alkylation or from the crude crystals of the product therefrom, and further N,N-di(3,3-dimethylbutyl)-APM as the by-product can be separated and removed by means for separation and purification, such as crystallization, chromatography, extraction, treatment with the activated charcoal, etc. As a result, N-[N-(3,3-dimethylbutyl)-L-&agr;-aspartyl]-L-phenylalanine methyl ester which is a sweetener having a high degree of sweetness can be purified efficiently, and also the APM derivative in a high purity is possibly produced industrially.