The purpose of the present invention is to reduce the size of a head up display device that has a reflector. This head up display device is provided, inside a casing thereof, with an image display; and a light path forming component (25) that has a reflector (29) that reflects an image (23), the reflector having a quadrangular basic shape. Here, a cut-away corner portion (42-44) is formed at at least one of four corner portions (31-34) of the reflector (29) having a quadrangular basic shape. Furthermore, the reflector (29) having a quadrangular basic shape is arranged inside the casing so as to be inclined with respect to the horizontal direction and the cut-away corner portion (42-44) is provided at least one of an uppermost corner portion (32) and a lowermost corner portion (34) that are at diagonally opposite positions on the reflector (29) which is arranged so as to be inclined.