The present invention addresses the problem of providing: a posterior silk gland gene expression unit, whereby a recombinant peptide, in particular a water-soluble peptide, can be expressed in a large amount in the posterior silk gland of a silkworm such as Bombyx mori； and a transgenic silkworm into which the aforesaid gene expression unit has been transferred. Provided is a posterior silk gland gene expression unit which comprises: a promoter of a gene encoding a protein, said protein being expressed specifically in the posterior silk gland； a DNA encoding a signal peptide originating in a protein, said protein being expressed specifically in the middle silk gland, functionally bound to the downstream of the promoter； and a DNA encoding a target peptide, said peptide being free from proteins secreted as constituents of fibroin, ligated to the downstream thereof.
