The present invention is to provide a plant and a plant storage organ thereof in which GLP-1 derivatives are accumulated, and a method of producing them in order to develop a method for ingesting orally GLP-1 derivatives at a low cost and to make use of it in diabetic treatment. A transgenic plant or a plant storage organ thereof in which GLP-1 derivatives are accumulated cleavable with a digestive enzyme is produced by a method comprising: integrating into a vector a linked GLP-1s-DNA which comprises tandem repeated "n" DNAs ("n" being an integer of 3 or more) encoding a GLP-1 derivative consisting of GLP-1 (7-36) or of an amino acid sequence of GLP-1 (7-36) in which one or a few amino acids are deleted, substituted and/or added, and C-terminal consists of Arg or Lys, and having GLP-1 activity; introducing the vector into a plant cell; and redifferentiating the obtained transformant. The transgenic plant and the plant storage organ are useful as a pharmaceutical composition or a food or drink for treating or preventing diabetes.