Disclosed is a vehicle-mounted electronic device wherein heat generated from a heat-generating electronic device can be efficiently released without redesigning various kinds of components even if an input/output signal array for a terminal of a connector is changed. The vehicle-mounted electronic device is comprised of a lower housing (2b) having a bottom surface, a part of which is opened; a connector (3) which is projected from an opening (2c) of the lower housing (2b), and which can be connected to an outside device which inputs/outputs electric signals and an outside power source which supplies electric power; a circuit board (5) which is provided within the lower housing (2b), and is electrically connected to the connector (3) to process signals using electric signals and electric power; a metal wiring plate (7) composed of a plurality of metal wires, which is provided within the lower housing (2b) on the side adjacent to the bottom surface of the lower housing (2b), and has an end portion projected from the lower housing (2b) to electrically connect to the circuit board; and heat-generating electronic components (6), the bottom surfaces of which are coupled to a part of the metal wires in the metal wiring plate (7). The thickness of the metal wires coupled to the bottom surfaces of the heat-generating electronic components (6) in the metal wiring plate (7) is larger than the thickness of the terminal of the connector (3).