A cured product of a resin composition, the cured product comprising at least silsesquioxane being a polymer of silsesquioxane having an oxetanyl group, wherein when the average thermal expansion coefficient of the cured product at 30 to 200°C is expressed as α1 [K -1], at least one of requirements α1/α2 ≥ 10 and (α1 - α2) 2 × 10 8 ≥ 0.4 is satisfied, where the average thermal expansion coefficient α1 is determined according to Japanese Industrial Standards K7197-1991 and a reference value α2 is 350 × 10 -6 [K -1] or less, requirements 0.09 ≤ Ia/Ib ≤ 3.0, 0.04 ≤ Ic/Ib ≤ 1.0, Ia/Id ≥ 47, and Ie/Id ≥ 59 are satisfied when an absorbance derived from a siloxane bond in which two silicon atoms are bonded to one oxygen atom, an absorbance derived from a hydrocarbon group, and an absorbance derived from a hydroxy group, an absorbance derived from the oxetanyl group, and an absorbance derived from a bond between a silicon atom and a non-reactive functional group are respectively expressed as Ia, Ib, Ic, Id, and Ie, the absorbances Ia, Ib, Ic, Id, and Ie are determined by attenuated total reflection using a Fourier transform infrared spectrophotometer, the absorbance Ia is determined by subtracting an absorbance on a baseline of the infrared absorption spectrum and at a wavenumber where the local maximum of absorbance in an absorption band derived from the siloxane bond appears from the local maximum of absorbance in the absorption band derived from the siloxane bond, the absorbance Ib and the absorbance Ic are respectively determined as the local maximum of absorbance of the infrared absorption spectrum in an absorption band derived from a hydrocarbon group and the local maximum of absorbance of the infrared absorption spectrum in an absorption band derived from a hydroxy group, the absorbance Id is determined by subtracting an absorbance on a baseline of the infrared absorption spectrum and at a wavenumber where the local maximum of absorbance in an absorption band derived from the oxetanyl group appears from the local maximum of absorbance in the absorption band derived from the oxetanyl group, and the absorbance Ie is determined by subtracting an absorbance on a baseline of the infrared absorption spectrum and at a wavenumber where the local maximum of absorbance in an absorption band derived from the bond between a silicon atom and a non-reactive functional group appears from the local maximum of absorbance in the absorption band derived from the bond. when the average thermal expansion coefficient of the cured product at 30 to 200°C is expressed as α1 [K -1], at least one of requirements α1/α2 ≥ 10 and (α1 - α2) 2 × 10 8 ≥ 0.4 is satisfied, where the average thermal expansion coefficient α1 is determined according to Japanese Industrial Standards K7197-1991 and a reference value α2 is 350 × 10 -6 [K -1] or less, requirements 0.09 ≤ Ia/Ib ≤ 3.0, 0.04 ≤ Ic/Ib ≤ 1.0, Ia/Id ≥ 47, and Ie/Id ≥ 59 are satisfied when an absorbance derived from a siloxane bond in which two silicon atoms are bonded to one oxygen atom, an absorbance derived from a hydrocarbon group, and an absorbance derived from a hydroxy group, an absorbance derived from the oxetanyl group, and an absorbance derived from a bond between a silicon atom and a non-reactive functional group are respectively expressed as Ia, Ib, Ic, Id, and Ie, the absorbances Ia, Ib, Ic, Id, and Ie are determined by attenuated total reflection using a Fourier transform infrared spectrophotometer, the absorbance Ia is determined by subtracting an absorbance on a baseline of the infrared absorption spectrum and at a wavenumber where the local maximum of absorbance in an absorption band derived from the siloxane bond appears from the local maximum of absorbance in the absorption band derived from the siloxane bond, the absorbance Ib and the absorbance Ic are respectively determined as the local maximum of absorbance of the infrared absorption spectrum in an absorption band derived from a hydrocarbon group and the local maximum of absorbance of the infrared absorption spectrum in an absorption band derived from a hydroxy group, the absorbance Id is determined by subtracting an absorbance on a baseline of the infrared absorption spectrum and at a wavenumber where the local maximum of absorbance in an absorption band derived from the oxetanyl group appears from the local maximum of absorbance in the absorption band derived from the oxetanyl group, and the absorbance Ie is determined by subtracting an absorbance on a baseline of the infrared absorption spectrum and at a wavenumber where the local maximum of absorbance in an absorption band derived from the bond between a silicon atom and a non-reactive functional group appears from the local maximum of absorbance in the absorption band derived from the bond.