A portable electronic device of the present invention includes a package, an electronic device body, a battery housing portion including a battery fitting portion for fitting a secondary battery therein, a foaming agent-containing layer provided on a surface of the battery housing portion, and a secondary battery to be fitted in the battery fitting portion. The foaming agent-containing layer is normally thin and, in the event of the battery generating heat, the layer turns into a thicker foamed layer which exhibits superior thermal insulation properties. Thus, it is possible to provide a portable electronic device having the degree of freedom in design for size and thickness reduction achieved at a high level, while offering high-level safety and reliability.