An external mounting type head up display device for a vehicle, which can be mounted irrespective of kind of vehicle, is disclosed. The head up display device includes a display unit to convert a signal into an image and output the image, an optical element decentered with respect to an optical axis of the display unit, the optical element magnifying the image and transmitting the image, and a combiner provided in front of a driver to provide the driver with the transmitted magnified image.