A method for accumulating a protein in a plant cell, in which by co-expressing a gene which encodes a polypeptide having an ability to form an ER body, and a gene which encodes a target protein having an endomembrane system migration signal peptide at the N-terminus and having an ER retention signal peptide at the C-terminus in the plant cell, the target protein or a protein in which the N-terminus domain of the target protein is lacking is accumulated in an ER body formed in the plant cell.