As a conventional vehicle warning device, there has been known a device, which issues a warning using a headup display unit as disclosed in Patent Literature 1. The vehicle warning device disclosed in Patent Literature 1 detects a distance between an own vehicle and a preceding vehicle located ahead of the own vehicle, superimposes a mark image on a preceding vehicle in a visual field of a vehicle driver (user) by a headup display unit, and displays an index image numerically showing a distance between the own vehicle and the preceding vehicle in other area than the mark image by a headup display unit. Therefore, it is possible to inform a user of a position of an object (preceding vehicle) to be paid attention by displaying a mark image, and to inform a user of a dangerous index (vehicle distance) by displaying a dangerous index image.
However, as the mark image and the dangerous index image denoting a vehicle distance and the like are displayed at positions distant from each other, a user always has to move eyes to recognize the position of the object to be paid attention and the dangerous index. This might distract the attention of the user.
Further, Patent Literature 2 discloses a vehicle driving assist device, which calculates a risk potential such as a distance between an own vehicle and a preceding vehicle, and changes the size, color, brightness and the like of a mark image to be superimposed on a preceding vehicle in accordance with the risk potential.