The present invention provides a thermosetting resin composition comprising (A) a curing agent having an acidic substituent and an unsaturated maleimide group which is produced by a specific method, (B) a 6-substituted guanamine compound and/or dicyandiamide, (C) a copolymer resin comprising specific monomer units and (D) an epoxy resin and a prepreg and a laminated plate which are prepared by using the same. The thermosetting resin composition of the present invention is balanced in all of a copper foil adhesive property, a heat resistance, a moisture absorption, a flame resistance, a metal-stuck heat resistance, a relative dielectric constant and a dielectric loss tangent. They have a low toxicity and are excellent in a safety and a working environment, and therefore a prepreg and a laminated plate which have excellent performances are obtained by using the above thermosetting resin composition.