The present invention is regarding plants and plant storage organs thereof in which GLP-1 derivatives are accumulated, and methods of producing them. The transgenic plants and plant storage organs thereof accumulate tandem repeated GLP-1 derivatives cleavable with intestinal digestive enzyme to monomeric molecules and are produced by methods comprising: integrating into vectors linked DNAs which comprise tandem repeated DNAs encoding the GLP-1 derivative with trypsin resistance in which the amino acid in the 26th position is Gln, the amino acid in the 34th position is Asn or Asp, and C-terminal consists of Arg or Lys to produce monomeric molecules; introducing the vectors into plant cells; and redifferentiating the obtained transformants. The edible transgenic plants and plant storage organs are useful for treating diabetes and can be ingested by diabetic patients.