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