A head-up display device includes a projector, an expansion optical system, an estimating part, a determination part, and a light amount control part. The projector has a light source and a liquid crystal panel. The expansion optical system expands an imagery output from the projector. The estimating part estimates a relative position of the sun with respect to a mobile unit. The determination part determines whether the liquid crystal panel is protected based on a relation between the relative position estimated by the estimating part and a solid angle representing a range in which a light from the sun is incident to the liquid crystal panel through the expansion optical system. The light amount control part decreases an amount of light incident to the liquid crystal panel when the determination part determines that the liquid crystal panel should be protected.