Head-up display device

A head-up display device, comprising:nan indicator configured to emit a display light;a lower casing to which the indicator is mounted;an upper casing which is mounted to an upper portion of the lower casing;a pair of support portions each having a first end provided on the lower casing and a second, opposite end facing the upper casing;guide grooves which are formed on the support portions at opposite sides thereof, wherein the guide grooves respectively have insertion ports which are open at the second end of the corresponding support portions; anda reflective mirror which is inserted into the insertion ports of the guide grooves toward a bottom surface of the lower casing while both lateral edges of the reflective mirror are engaged with the guide grooves and mounted to the lower casing, to reflect the display light emitted from the indicator and guide the reflected display light to a windshield of a vehicle.