The composite filler filler forms a core-shell structure composed of a core substance made of carbon, metal, zinc oxide or zirconium oxide and a shell substance attached to a surface of the core substance. The shell substance is formed from fumed oxide particles, the fumed oxide particles being attached to a part or a whole of the surface of the core substance by mixing of the fumed oxide particles and the core substance in a dry ball mill. The fumed oxide particles are particles changed from bulky aggregated particles to gathered bulk particle. In the composite filler, percentage of the core substance ranges from 30 vol % to 85 vol % and percentage of the shell substance ranges from 15 vol % to 70 vol %, a thermal conductivity is 0.075 W/m.Math.K or more, a volume resistivity is 1.010.sup.5 .Math.cm or more, and a dielectric breakdown voltage is 1 kV/mm or more.

The composite filler filler forms a core-shell structure composed of a core substance made of carbon, metal, zinc oxide or zirconium oxide and a shell substance attached to a surface of the core substance. The shell substance is formed from fumed oxide particles, the fumed oxide particles being attached to a part or a whole of the surface of the core substance by mixing of the fumed oxide particles and the core substance in a dry ball mill. The fumed oxide particles are particles changed from bulky aggregated particles to gathered bulk particle. In the composite filler, percentage of the core substance ranges from 30 vol % to 85 vol % and percentage of the shell substance ranges from 15 vol % to 70 vol %, a thermal conductivity is 0.075 W/m.Math.K or more, a volume resistivity is 1.010.sup.5 .Math.cm or more, and a dielectric breakdown voltage is 1 kV/mm or more.