A semiconductor device with improved reliability and its manufacturing method is offered. The semiconductor device of this invention includes a pad electrode formed on a semiconductor substrate through a first insulation layer, and a via hole formed in the semiconductor substrate and extending from a back surface of the semiconductor substrate to the pad electrode, wherein the via hole includes a first opening of which a diameter in a portion close to the pad electrode is larger than a diameter in a portion close to the back surface of the semiconductor substrate, and a second opening formed in the first insulation layer and continuing from the first opening, of which a diameter in a portion close to the pad electrode is smaller than a diameter in a portion close to the front surface of the semiconductor substrate.