A method for the improving thermal efficiency of a heating device, comprising: installing a heat-resistant inorganic conjugated molded product in and along a pathway for heated gas generated from a heating device without interrupting a flow of heated gas passing the pathway; heating the inorganic conjugated molded product with the heated gas; putting back into the heating device radiation heat from the heated inorganic conjugated molded product; and reducing an amount of heat that flows out from the heating device to the outside, the inorganic conjugated molded product being provided with an interior layer and an exterior layer, the exterior layer being formed of a coverture for inorganic materials that protects the interior layer from heated gas, characterized in that the interior layer is formed by a reinforcing material and a ceramic matrix, the reinforcing material being formed of a heat-resistant inorganic fiber, the ceramic matrix being filled in voids in the reinforcing material, and the reinforcing material is formed having a cloth member or a processed fiber product; the ceramic matrix is any one of an inorganic product obtainable by pyrolyzing polymetallocarbosilane, an inorganic product obtainable by pyrolyzing polycarbosilane, a pyrolysate of polyaluminocarbosilane, a pyrolysate of polycarbosilane, a product of carbonization reaction between molten silicon and carbon, a product of carbonization reaction between molten silicon and a carbon compound, a product of sintering reaction of a SiC fine powder containing a sintering additive, an inorganic product obtainable by pyrolyzing a silazane-based polymer containing Si and N, and Si 3 N 4 crystalline ultrafine particles which are products of sintering reaction of a Si 3 N 4 fine powder containing a sintering additive; and provided that each element of Al, Ti, Cr, Fe, Si, Co, Ni, Cu, Y, Zr, Nb, Tc, Ru, Rh, Pd, Ag, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Re, and Os is in a first group, the coverture for inorganic material that forms the exterior layer is composed of a material A consisting of any one of an oxide of one element selected from the first group, a complex oxide consisting of two or more elements selected from the first group, a solid solution oxide of two or more elements selected from the first group, the oxide and the complex oxide, the oxide and the solid solution oxide, the complex oxide and the solid solution oxide, and the oxide, the complex oxide, and the solid solution oxide, and a thickness of the exterior layer is 0.2 μm or more and 10 μm or less. installing a heat-resistant inorganic conjugated molded product in and along a pathway for heated gas generated from a heating device without interrupting a flow of heated gas passing the pathway; heating the inorganic conjugated molded product with the heated gas; putting back into the heating device radiation heat from the heated inorganic conjugated molded product; and reducing an amount of heat that flows out from the heating device to the outside, the inorganic conjugated molded product being provided with an interior layer and an exterior layer, the exterior layer being formed of a coverture for inorganic materials that protects the interior layer from heated gas, characterized in that the interior layer is formed by a reinforcing material and a ceramic matrix, the reinforcing material being formed of a heat-resistant inorganic fiber, the ceramic matrix being filled in voids in the reinforcing material, and the reinforcing material is formed having a cloth member or a processed fiber product; the ceramic matrix is any one of an inorganic product obtainable by pyrolyzing polymetallocarbosilane, an inorganic product obtainable by pyrolyzing polycarbosilane, a pyrolysate of polyaluminocarbosilane, a pyrolysate of polycarbosilane, a product of carbonization reaction between molten silicon and carbon, a product of carbonization reaction between molten silicon and a carbon compound, a product of sintering reaction of a SiC fine powder containing a sintering additive, an inorganic product obtainable by pyrolyzing a silazane-based polymer containing Si and N, and Si 3 N 4 crystalline ultrafine particles which are products of sintering reaction of a Si 3 N 4 fine powder containing a sintering additive; and provided that each element of Al, Ti, Cr, Fe, Si, Co, Ni, Cu, Y, Zr, Nb, Tc, Ru, Rh, Pd, Ag, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Re, and Os is in a first group, the coverture for inorganic material that forms the exterior layer is composed of a material A consisting of any one of an oxide of one element selected from the first group, a complex oxide consisting of two or more elements selected from the first group, a solid solution oxide of two or more elements selected from the first group, the oxide and the complex oxide, the oxide and the solid solution oxide, the complex oxide and the solid solution oxide, and the oxide, the complex oxide, and the solid solution oxide, and a thickness of the exterior layer is 0.2 μm or more and 10 μm or less.