The present invention presents construction of a detection method requiring no step of removing free biotin during preparation of a biotinylated protein having a biotin tag, in a detection method of a substance interacting with a protein, and studied various preparation methods of the biotinylated protein. In order to solve the above-mentioned problem, the present inventor has found that in a cell-free protein synthesizing system, in particular, a wheat embryo cell-free protein synthesizing system, when biotinylation is performed during or after protein's synthesis, the biotinylation of the protein can be attained in an remarkably lower concentration of the biotin than that in the conventional biotinylation operations, and has accomplished the present invention by using the protein having the biotin tag in each detection system.