There is provided a process for preparing a pharmacologically acceptable salt of N-(1(S)-ethoxycarbonyl-3-phenylpropyl)-L-alanyl-amino acid which comprises condensing an amino acid and N-(1(S) -ethoxycarbonyl-3-phenylpropyl)-L-alanine N-carboxy-anhydride under basic condition, carrying out decarboxylation under between neutral and acidic condition to obtain N-(1(S)-ethoxycarbonyl-3-phenylpropyl)-L-alanyl-amino acid, and forming a pharmacologically acceptable salt thereof, wherein the production of a by-product (3): is suppressed by carrying out in an aqueous liquid a series of operations till formation of the pharmacologically acceptable salt or till isolation of the pharmacologically acceptable salt. The present invention enables to prepare the pharmacologically acceptable salt of N-(1(S)-ethoxycarbonyl-3-phenylpropyl)-L-alanyl-amino acid having high quality, in a commercial scale with high yield and economical efficiency.