The present disclosure provides a composite including a nitride sintered body having a porous structure and a semi-cured product of a heat-curable composition impregnated into the nitride sintered body, wherein a dielectric breakdown voltage obtainable after disposing the composite between adherends, heating and pressurizing the composite for 5 minutes under the conditions of 200°C and 10 MPa, and further heating the composite for 2 hours under the conditions of 200°C and atmospheric pressure, is greater than 5 kV.