To protect surely an electric constituent portion from a potential having an undesired value, by coupling conductor portions to each other by the terminal of a protective element and a breaking position which are separated from each other.

Solder pads 11, 12 have a breaking position 13 whereby they are divided into two individual portions 11, 12 separated electrically from each other. In that case, the portion 11 is connected with a conductor path 5 led to a voltage supplying apparatus, and the portion 12 is connected with a conductor path 6 led to an electric constituent portion. Since a terminal 16 of a diode 4 bridges the breaking position 13 before a protective action is initiated, the solder pad portions 11, 12 are connected electrically with each other. When the protective action is initiated, the solders are liquidized by the heating of the diode 4. In this case, the bridging by the terminal 16 cannot be surely ensured already. As a result, the electric constituent portion is protected from an unallowable voltage value.