The present invention relates to a transformed plant cell expressing β-amyloid gene, which is known as a causal substance of senile dementia in human, a transformed plant and a preparation method of the same, more particularly, to a transformed plant cell expressing tandem repeats of β-amyloid gene, a transformed plant produced by tissue-culturing the above cells and a preparation method of the same. The transformed plant of the present invention expresses multiple β-amyloid protein, so that the plant can be effectively used for the preparation of an oral vaccine composition for the prevention of senile dementia in human.