Provided is a head-up display device that can detect background brightness with a simple constitution. A HUD device (1) is provided with: a display device (20) that outputs display light (L) that shows a display image; a combiner (50) that has a concave surface (50a) to which the display light (L) output by the display device (20) is incident, said combiner (50) concentrating the incident display light (L) so as to allow an observer (2) to see the display image from the concave surface (50a) side superimposed on a background from the front; and an optical sensor (31) that is positioned below the combiner (50) and detects the brightness of the light reaching that optical sensor (31). The combiner (50) has a light guide body (60) which is at the lower edge part thereof and is integrated with the combiner (50). The light guide body (60) has a facing surface (62) facing the optical sensor (31) in the vertical direction and guides external light incident from the front downward, outputting the guided light from the facing surface (62). The optical sensor (31) detects the brightness of the light output from the facing surface (62).