A nitride-based ceramics resin composite body having thermal conductivity, electrical insulation, and adhesion to adherends equal to conventional products, and having improved heat resistance reliability during the reflow process, and a thermal conductive insulating adhesive sheet using the same are provided. A nitride-based ceramics resin composite body in which a thermosetting resin composition is impregnated in a porous nitride-based ceramics sintered body is provided. The thermosetting resin composition includes a specific epoxy resin and a bismaleimide triazine resin, and a water absorption of the thermosetting resin composition in a completely cured state measured in accordance with method A in JIS K7209 (2000) is 1% by mass or less.