This invention provides a latent catalyst having a structure of phosphonium borate consisting of a monovalent cation portion in which four specific groups are bonded to the phosphorus atom and a monovalent anion portion in which four specific groups are bonded to the boron atom, and a latent catalyst having a structure wherein the above phosphonium borate is the recurring unit and at least two of said recurring unit are connected through at least one of the four specific groups bonded to the boron atom. This invention also provides a thermosetting resin composition comprising such a latent catalyst and an epoxy resin molding material comprising such a latent catalyst and further provides a semiconductor device in which a semiconductor is encapsulated with said epoxy resin molding material.