According to the present invention, a method for stably accumulating a target protein in plant cells or a plant body, and a transgenic plant in which protein has accumulated, are provided. The method of the present invention is a method for accumulating protein in plant cells, comprising accumulating a target protein or a protein deficient in an N-terminal region of the target protein in vacuoles of myrosin cells present in a multiple mutant, in which myrosin cells deficient in intravacuolar protein are also present in a plant body at locations other than around vascular bundles, by expressing a gene that encodes a target protein having an intracellular membrane system localization signal on the N-terminal and a vacuole localization signal on the C-terminal in the multiple mutant.