Provided are: a novel bonds useful as a highly-functional material; and a novel production method for a polycrystalline sintered product of a bonds, of which the energy cost is low, which does not require a sintering promoter, which enables the product to be worked into complicated forms and which enables a development to a polynary boride.Provided are a boride having a composition Na—Si—B, and a polycrystalline reaction-sintered product thereof. A mixed compact of boron and an element, M (M means Si and/or C) is heated along with metal sodium to give a polycrystalline reaction-sintered product.