A power module base 1 includes a heat radiation substrate 2 formed of a high-thermal-conduction material, an insulating substrate 3 joined to an upper surface of the heat radiation substrate 2, a wiring layer 8 provided on an upper surface of the insulating substrate 3, and a heat radiation fin 4 joined to a lower surface of the heat radiation substrate 2. A component attachment plate 5 which is thicker than the heat radiation substrate 2 and has a through hole 11 for accommodating the insulating substrate 3 is joined to the upper surface of the heat radiation substrate 2 such that the insulating substrate 3 is located within the through hole 11. This power module base 1 can maintain the upper surface of the component attachment plate 5 flat, and various components required for a power module, such as a casing 13, can be attached onto the component attachment plate 5.