Because the thermal conductive medium 9 causes thermal conduction also by convection, the thermal conductive medium 9 can efficiently dissipate heat absorbed from the temperature-rising portions over the entire heat spreader 6.

As the liquid thermal conductive medium 9, alcohol, water, glycerin, or the like can be preferably used. Alternatively, one of various sols, such as hydrosol, organosol, and alcosol, can be used.

The heat spreader 6 according to the embodiment is flexible and therefore does not require a large external force to be “deformed to conform to the three-dimensional shape on the board”. Therefore, because deformation of the heat spreader does not involve application of an excessively large mechanical force to the electronic elements and the like, damages to the electronic elements and the like will not occur.

Furthermore, because the thermal conductive medium 9 is enclosed in the package 8, favorable repair workability is provided. Specifically, the thermal conductive medium 9 can be taken out easily in one piece with the package 8 at repair.

Because the heat spreader 6 deforms to conform to the three-dimensional shape on the board, causing the thermal conductive medium 9 to spread all over the package 8, it is possible to load “a maximum fillable amount of the thermal conductive medium” in the electronic device.

When built into the electronic device, the heat spreader 6 deforms to conform to the three-dimensional shape on the circuit board. However, after once built in and deformed, the heat spreader will not deform any more. Accordingly, the heat spreader is not required to be “capable of deforming” after being built in.