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.