Transformation of a model legume Lotus japonicus accession Miyakojima MG-20 was examined using Agrobacterium tumefaciens with a binary expression vector. Using the improved transformation method, we introduced a major allergen gene from a house dust mite, Der f 1, into MG-20. Analyses by Southern hybridization, reverse transcription (RT)-PCR, and Western blotting showed that the Der f 1 gene was integrated into the genome of L. japonicus, expressing the gene product in the T1 lines. Our results imply future application of oral allergen-specific immunotherapy using legume plants.