A silicon nitride substrate comprises a substrate comprising a silicon nitride sintered body, and a plurality of granular bodies containing silicon and integrated to a principal surface of the substrate, wherein a plurality of needle crystals or column crystals comprising mainly silicon nitride are extended from a portion of the granular bodies. A brazing material is applied to a principal surface of the substrate, and a circuit member and a heat radiation member are arranged on the applied brazing material, and bonded by heating. Because of a plurality of granular bodies integrated to the principal surface of the substrate, and a plurality of the needle crystals or the column crystals extended from a portion of the granular bodies, a high anchor effect is produced so that the circuit member and the heat radiation member are firmly bonded to the silicon nitride substrate.