An electronic part mounting method, a semiconductor module, and a semiconductor device, which can reduce a mounting area and a device thickness. In an electronic part mounting method for bonding an electrode formed on a substrate and an electrode formed on an electronic part to each other, the method comprises the step of bonding both the electrodes through a metal layer made up of aggregated particles of at least one kind of metal. Then, the metal particles have an average particle size of 1 to 50 nm. Preferably, the metal particles form a metal layer having a thickness of 5 to 100 μm.