Transgenic silkworms comprising GFP whose expression is regulated by the sericin gene promoter were produced. Observation of the silk glands of the last instar larvae of the silkworms showed fluorescence only in the middle silk glands. GFP was secreted from middle silkgland cells from around the spinning stage, indicating that GFP moved into the gland lumen. Finally, GFP was spun into cocoon filaments, and cocoons containing large amounts of GFP were produced. Thus, by using the promoter region of the sericin gene, recombinant proteins can be produced in the middle silk glands. Furthermore, the recombinant proteins produced in the middle silk glands were readily secreted into the lumen of the middle silk glands.