To increase the reliability of a product by preventing the deterioration in dielectric strength of a control circuit board mounted on a metallic base caused by application of voltage of a dielectric strength test level and preventing dielectric breakdown in circuit elements due to partial discharging.

A semiconductor device is built up by assembling a power circuit block 2 and a control circuit block 3 made by mounting circuit elements 3b on a printed board 3a, in an outer packaging case which is combined with a metallic base board 1 for radiation. The printed board 3a of the control circuit block 3 is mounted on the metallic base board 1 side by side with the power circuit block 2. On a rear face of the printed board 3a, a metallized layer 3c is formed. When mounting the printed board 3a on the metallie base board 1, the printed board 3a is joined to the metallic base board 1 by soldering or with a conductive adhesive so that the metallized layer 3c may be electrically connected to the metallic base board 1. In order to prevent the appearance of partial discharging at the junction between the two components, the rear face of the printed board 3a and the metallic base board 1 are made at the same potential.