There is provided a thermosetting light-reflecting resin composition that has a high level of various characteristics required of optical semiconductor element mounting boards, such as optical properties and thermal discoloration resistance, provides high releasability during molding such as transfer molding, and allows molding processes to be performed continuously. There are also provided a highly-reliable optical semiconductor element mounting board and an optical semiconductor device each produced with the resin composition, and methods for efficient production thereof. A thermosetting light-reflecting resin composition is prepared and used, which includes (A) an epoxy resin, (B) a curing agent, (C) a curing catalyst, (D) an inorganic filler, (E) a white pigment, (F) an additive, and (G) a release agent as major components, wherein the resin composition, after curing, has a diffuse reflectance of 80% or more at a light wavelength of 400 nm; and the resin composition is possible to perform transfer molding 100 times or more continuously.