A method of manufacturing a semiconductor device includes: forming a first layer including crystals by processing a surface of a first electrode of a semiconductor element; forming a second layer including crystals by processing a surface of a second electrode of a mounting member on which the semiconductor element is mounted; reducing a first oxide film present over or in the first layer and a second oxide film present over or in the second layer at a first temperature, the first temperature being lower than a second temperature at which a first metal included in the first electrode diffuses in a solid state and being lower than a third temperature at which a second metal included in the second electrode diffuses in a solid state; and bonding the first layer and the second layer to each other by solid-phase diffusion.