A portable electronic device according to the present invention is provided with a casing formed by joining two cabinets together. A housing chamber for an electric component is recessed on at least either one cabinet and is open toward an inner face of the other cabinet. Mating surfaces located on both the cabinets overlap each other around the housing chamber. A water entry portion leading to a clearance between the mating surfaces is located on an exposed surface exposed to the outside of the casing in an upper part of the casing. A groove extending in a right-left direction along the hosing chamber is recessed on the mating surface of at least either one cabinet of the two cabinets at a position away upward from the housing chamber.