A method for accumulating a protein in a plant cell, said method comprising co-expressing in the plant cell a gene, which encodes a polypeptide capable of forming an ER body, with a gene, which encodes a target protein having, at the N-terminus thereof, an intracellular membrane system localization signal peptide and also having, at the C-terminus thereof, an ER retention signal peptide, and thus accumulating said target protein or a protein lacking the N-terminal domain of the target protein within the ER body that is formed in said plant cell.
