A multi-layer wiring substrate includes an aluminum nitride ceramic substrate, a multi-layer wiring part having an electric insulating layer of an organic polymer, a die pad for mounting thereon an electronic part, and a thermal via of a column shape for effectively dissipating heat generated in the mounted electric part. The multi-layer wiring part is integrally formed on the ceramic substrate. The die pad is provided on a surface of the multi-layer wiring part. One end of the thermal via connects to the die pad, while the other end of the thermal via passes through the multi-layer wiring part and extends at least to the ceramic substrate. The thermal via is electrically insulated from the multi-layer wiring part. In the second multi-layer wiring substrate, at least one of end portions of the above thermal via is wider than a section of the other portion of the thermal via, resulting in further improvement of the heat dissipating property.