Provided is a vehicle-mounted electronic control device including a control unit reduced in size, weight, and costs with improved contact reliability between electric components included in the control unit. In the vehicle-mounted electronic control device according to the present invention, a connector has a locking hole formed in a side surface relative to an upper surface that is an upward oriented surface, a fitting opening formed in a lower surface, and a waterway for preventing flowing down water from entering the locking hole and the fitting opening, which is formed on a surface of the connector.