A method for preparing a phenylalanine derivative having a quinazoline-dione ring represented by the following formula (1) or a pharmaceutically acceptable salt thereof, comprising the following steps (a), (b) and (c): (a) reacting an acyl phenylalanine derivative represented by the following formula (2): with a carbonyl group-introducing reagent and a specific anthranilic acid derivative to thus form the corresponding carboxy-asymmetric urea derivative; (b) converting the carboxy-asymmetric urea derivative into the corresponding quinazoline-dione derivative in the presence of a carboxyl group-activating agent: (c) if desired, substituting an N-alkyl group for the hydrogen atom bonded to the nitrogen atom present in the quinazoline-dione ring of the quinazoline-dione derivative using an N-alkylation agent and then deprotecting the resulting product, when the substituent R3′ which is a group corresponding to R3 is protected. According to this method, there can be obtained a phenylalanine derivative having a quinazoline-dione skeleton in a industrially favorably high yield.