A semiconductor device manufacturing method includes a step of preparing a semiconductor unit, having a first main surface including a heat releasing portion and a second main surface opposite to the first main surface, in which is mounted a semiconductor chip, a step of preparing a cooler having a flat surface, a step of applying a paste including metal nanoparticles to the first main surface of the semiconductor unit or the flat surface of the cooler, a step of bringing the first main surface of the semiconductor unit and the flat surface of the cooler into contact through the paste, and a step of applying a pressurizing force uniform in-plane to the second main surface of the semiconductor unit at the same time as raising the temperature of the paste, thereby sintering the paste and forming a junction layer.