Abstract: A portable electronic device has a casing has an upper case body and a rear case cover both made of a thermally conductive material. A heat insulating member thermally insulates the rear case cover from the upper case body of the casing. A thermoelectric generator unit is disposed in the casing for generating an electromotive force based on the Seebeck effect. The thermoelectric generator unit has a heat radiating plate for transferring heat to the upper case body and a heat absorbing plate. A heat conductive spacer is made of a thermally conductive and elastically deformable sheet of material and has opposed main surfaces. One of the main surface is disposed in contact with the heat absorbing plate of the thermoelectric generator unit and the other main surface is disposed in contact with an inner side surface of the rear case cover for conducting heat from the rear case cover to the thermoelectric generator unit.