Electrically conductive graphite powder may be produced from the graphite sheet. It is likely that the graphite powder adheres to circumjacent electronic circuit boards and causes a failure, such as a short circuit. The graphite sheet is sandwiched between first insulating sheet 13 and second insulating sheet 14 having larger sizes than the graphite sheet and are sealed outside the outer periphery of the graphite sheet. This structure prevents scatter of the graphite powder and occurrence of a failure of the circumjacent electronic circuits.

Hole diameter D1 of first through hole 15 is about 3 mm, and hole diameter D2 of second through hole 16 is about 0.3 mm

The thermal conduction property decreases as the size of first through hole 15 becomes larger. Therefore, the size of first through hole 15 may be determined such that second through hole 16 can be formed and that inner wall surface 17 of first through hole 15 can be sealed with insulating lamination portion 18. Sealing with insulating lamination portion 18 may be made such that the graphite powder generated from inner wall surface 17 does not scatter to the outside. Although insulating lamination portion 18 is required to seal inner wall surface 17, insulating lamination portion 18 may not necessarily be adhered to inner wall surface 17.

Hole diameter D1 of first through hole 15 may preferably be equal to or larger than 1.0 mm to allow insulating lamination portion 18 to preferably seal inner wall surface 17 of first through hole 15. Hole diameter D1 of first through hole 15 may preferably be equal to or smaller than 3 mm to suppress reduction of the thermal conduction property.