To keep a thermal resistance constant without being affected by compressivility, by covering a facing surface and a part of a side surface or a front side surface of a cushioning sheet piece with a metal foil.

With upper and lower surfaces (facing surfaces) 11 and 12 and one side surface 13 of a cushioning sheet piece 1 covered, the cushioning sheet piece 1 and a metal foil 2 are bonded together with a bonding agent. Connection between a heat-generating semiconductor chip 4 mounted on a substrate 3 and a cabinet 5 is thermally conductive, and a conduction thermal resistance R is provided by R=rX/(r+X) where r is thermal resistance with a metal foil and X is thermal resistance with a cushioning sheet piece. A thermal resistance r is provided by r=r1+r2 where r1 is contact thermal resistance at an interface between upper/lower flat surface and opposite surface and r2 is series thermal resistance at a side surface metal foil part. Since the series thermal resistance r2 is almost constant even when the compresivility of cushioning thermal conductive material changes while the contact thermal resistance r1 is almost constant, the thermal resistance r is constant without being affected by compressivility.