A composite sheet includes a thermally-conductive sheet, a first insulating sheet that covers one surface of the thermally-conductive sheet, and a second insulating sheet that covers another surface of the thermally-conductive sheet, and seals the thermally-conductive sheet between the first and second insulating sheets. The thermally-conductive sheet has a first through hole provided therein. The composite sheet has an insulating lamination portion at which the first insulating sheet is stacked on the second insulating sheet to seal an inner wall surface of the first through hole. The insulating lamination portion has a second through hole provided inside the first through hole and having a hole diameter which is smaller than a hole diameter of the first through hole.