It is provided an insulating substrate including through holes for conductors arranged in the insulating substrate. A thickness of the insulating substrate is 25 to 100 μm, and a diameter of the through hole is 20 to 100 μm. The insulating substrate includes a main body part and exposed regions exposed to the through holes and is composed an alumina sintered body. A relative density of the alumina sintered body is 99.5 percent or higher. The alumina sintered body has a purity of 99.9 percent or higher, and has an average grain size of 3 to 6 μm in said main body part. Alumina grains are plate-shaped in the exposed region and the plate-shaped alumina grains have an average length of 8 to 25 μm.