In 2000, an another innovative method for generating transgenic silkworms was established. Since then, various approaches using transgenic silkworms have been applied to clarify the biological functions of silkworms as well as to develop industrial applications. In the industrial applications, transgenic silkworms have mostly been used for producing new high-performance silks or useful recombinant proteins. In addition, researchers have been developing a silkworm as a novel model animal for testing medicines or as a new biosensor. Some of the recombinant proteins already have been in commercial use, and a new apparatus and a large-scale plant for mass rearing of silkworm were constructed. At last, farmers started to rear the transgenic silkworms that produce a modified silk. This chapter focuses on the development of transgenic silkworm and its applications. The author introduces the methods of constructing transgenic silkworm by germline transformation using transposon vector or genome editing. Next, the gene expression systems by the transgenic silkworms, in addition to the applications in analyses of gene functions, modification of gene functions, productions of recombinant proteins and productions of modified silks are described. The trials for large-scale rearing of transgenic silkworms in new industrial sericulture is also introduced.