A head-up display device is for projecting display light onto a projection member. The head-up display device includes a display, a reflecting member, an outside light prevention portion, and a state controller. The display emits the display light. The reflecting member reflects the display light emitted from the display in a reflection direction to project the display light onto the projection member. The outside light prevention portion changes between an entering state in which outside light is able to enter the display and a preventing state in which outside light is prevented from entering the display. The state controller controls the outside light prevention portion to be (i) in the entering state when the display light is projected onto the projection member and (ii) in the preventing state when the display light is not projected onto the projection member.