Embodiments of the invention provide a semiconductor device having high reliability as they ease the thermal stress or a heat distortion or strain occurring during the manufacturing process or during operation, and the embodiments function with stability for a long time. A semiconductor device has a semiconductor substrate, an insulating ceramic plate on which the semiconductor substrate is mounted and stress buffer 40 that eases a thermal stress. The stress buffer is provided between the semiconductor substrate and the insulating ceramic plate and can be provided on a surface of the insulating ceramic plate that is opposite to a surface on which the semiconductor substrate is mounted. The stress buffer is formed from a structure including at least Al and a second phase. The second phase is Al4X where X is at least one element of alkaline earth metal elements.