A composite sheet 10 includes porous a nitride sintered body 20 having a thickness of less than 2 mm and resins filled in pores of the nitride sintered body 20, and has a main surface 10a (10b) having a maximum height roughness Rz of less than 20 µm. A method for manufacturing the composite sheet 10 includes an impregnating step of impregnating pores of a porous the nitride sintered body 20 having a thickness of less than 2 mm with a resin composition, a smoothing step of smoothing the resin composition attached to a main surface of the nitride sintered body 20 to obtain a resin-impregnated body in which a part of the main surface is exposed, and a curing step of heating the resin-impregnated body to cure or semi-cure the resin composition impregnated in the pores to obtain the composite sheet 10.