A headup display device includes a driving mirror configured to reflect a display light irradiated from an indicator to guide the reflected display light to a windshield of a vehicle. The driving mirror includes a base plate fixed to a bottom surface of a casing, a pair of opposite support plates which are upright on the base plate, a holder pivotally supported by the opposite support plates, a reflective mirror held by the holder, a driving motor configured to pivot the holder to change an angle of the reflective mirror, and a positioning projection provided on the support plate and protruding in a mounting direction of the base plate to the bottom surface of the casing. A holding recessed portion is formed on the casing to allow an insertion of the positioning projection therein and hold the positioning projection at a predetermined position.