Second insulating sheet 14 includes base layer 14a made of an insulating film, and adhesive layer 14b stacked on base layer 14a at a first surface 14d of insulating sheet 14 facing thermally-conductive sheet 12. Second insulating sheet 14 further includes adhesive layer 14c stacked on base layer 14a at a second surface 14e of insulating sheet 14 opposite to the first surface 14d. Base layer 14a is made of a polyethylene terephthalate insulating film having a thickness of about 10 μm. Each of adhesive layer 14b and adhesive layer 14c is made of an acrylic adhesive layer having a thickness of about 10 μm. Second insulating sheet 14 is bonded to thermally-conductive sheet 12 and first insulating sheet 13 via adhesive layer 14b.

Inside first through hole 15, first insulating sheet 13 and second insulating sheet 14 are adhered to each other with adhesive layer 13b and adhesive layer 14b to form insulating lamination portion 18. Insulating lamination portion 18 seals inner wall surface 17 of first through hole 15.