Abstract: The present invention provides a silicon nitride sintered substrate capable of reducing contamination caused by a boron nitride powder or the like used as a releasing agent and problems in bonding strength and dielectric strength at the time of laminating metal layers or the like, where the contamination is caused by a network structure provided by a silicon nitride crystal formed on the surface of the substrate in an unpolished state after sintering a silicon nitride powder. The silicon nitride substrate in an unpolished state after sintering is a silicon nitride sintered substrate where a cumulative volume of pores having a diameter in a range of 1 to 10 ?m is not more than 7.0'10?5 mL/cm2 in a measurement by a mercury porosimetry. Preferably, Ra of the surface is not more than 0.6 ?m and arithmetic mean peak curvature (Spc) of a peak is not more than 4.5 [l/mm].