A translation template comprising an ORF region coding for a protein, a 5′ untranslated region comprising a transcription promoter and a translation enhancer and locating on the 5′ side of the ORF region, and a 3′ end region comprising a poly-A sequence and locating on the 3′ side of the ORF region, is expressed in a translation system in the presence of an agent for modifying a C-terminal of a protein, which comprises an acceptor portion having a group capable of binding to a protein through a transpeptidation reaction in a protein translation system and a modifying portion comprising a nonradioactive modifying substance linked to the acceptor portion via a nucleotide linker, to cause protein synthesis and the synthesized protein is purified. Thus, the yield of modified protein in a method of modifying C-terminal of protein is improved and detection of protein interaction based on various intermolecular interaction detection methods is realized at an improved level.