To suppress a flat surface to which a heat-generating body is mounted, from being expanded and curved due to an increase in internal pressure in a flat heat pipe.

A flat heat pipe 2 includes: a pair of flat plates (top plate 3 and bottom plate 4) in which a surface of one of the flat plates is formed into a mounting surface for a heat-generating body; a side plate 5 that surrounds peripheries of the pair of flat plates facing each other and defines a closed space therein; and an elastic body 7 for coupling the pair of flat plates to each other in the closed space 6. In the heat pipe 2, the side plate 5 is expanded and contracted so as to allow a variation in distance between the pair of flat plates. The side plate 5 typically has a bellows structure. When an internal pressure is increased in the heat pipe 2, the bellows structure of the side plate allows the distance between the pair of flat plates to be extended while the pair of flat plates are supported by the elastic body 7, thus the heat pipe 2 can suppress center parts of the plat plates from being curved.