A flame-retardant thermosetting resin composition comprising a thermosetting resin and from 4 to 50% by weight, based on the composition, of (1) melamine-coated ammonium polyphosphate particles and/or (2) water-insoluble ammonium polyphosphate particles obtained by crosslinking the surface of melamine-coated ammonium polyphosphate particles with a compound having a functional group capable of reacting with an active hydrogen atom belonging to an amino group of a melamine molecule in the melamine coating is disclosed. Also disclosed is a process for producing water-insoluble ammonium polyphosphate particles comprising adhering or impregnating a crosslinking agent having a functional group capable of reacting with an active hydrogen atom belonging to an amino group of a melamine molecule to or into the surface of melamine-coated ammonium polyphosphate particles and allowing a melamine in the melamine coat and the crosslinking agent to react.