The purpose is to develop and provide a method for easily and efficiently suppressing gene expression of sericin and/or fibroin H-chain, which are major silk gland-specific proteins, to efficiently produce a target protein in the silk glands as a protein mass production system in genetically-modified silkworms. Provided is a silkworm line capable of expressing within the silkworm antisense RNA having as a target site a repetitive sequence region of sericin and/or fibroin H-chain.