A head-up display device installed in a vehicle, comprising: a projector that projects a display light image representing vehicle-related information related to the vehicle; an optical unit that includes a drivable reflecting mirror, which reflects the display light image projected by the projector, and that adjusts a virtual image display position, where a virtual image of the display light image reflected by the reflecting mirror is displayed, by drive of the reflecting mirror; a stepping motor that rotates to drive the reflecting mirror; a reduction gear mechanism that rotatably includes a plurality of transmission gears (52, 53, 54, 55, 56, 57, 58, 59), which decelerate the rotation of the stepping motor and transmit the rotation to the reflecting mirror; a resilient member that generates a restoring force in a direction that the plurality of transmission gears (52, 53, 54, 55, 56, 57, 58, 59) are engaged with each other; and a control unit that controls a rotational position of the stepping motor in accordance with an adjustment command by an occupant of the vehicle, characterized in that: at least one of the plurality of transmission gears (52, 53, 54, 55, 56, 57, 58, 59) that is predicted to incur a creep deformation is defined as a specific gear; an environmental temperature of the specific gear, at which the specific gear is predicted to incur the creep deformation, is defined as a predicted deformation temperature (Te); a deviation predicted to be caused at the virtual image display position by the creep deformation at the predicted deformation temperature (Te) is defined as a predicted display deviation (δDe); and the control unit corrects the rotational position in a direction to reduce the predicted display deviation (δDe) when the environmental temperature rises to the predicted deformation temperature (Te) or higher. a projector that projects a display light image representing vehicle-related information related to the vehicle; an optical unit that includes a drivable reflecting mirror, which reflects the display light image projected by the projector, and that adjusts a virtual image display position, where a virtual image of the display light image reflected by the reflecting mirror is displayed, by drive of the reflecting mirror; a stepping motor that rotates to drive the reflecting mirror; a reduction gear mechanism that rotatably includes a plurality of transmission gears (52, 53, 54, 55, 56, 57, 58, 59), which decelerate the rotation of the stepping motor and transmit the rotation to the reflecting mirror; a resilient member that generates a restoring force in a direction that the plurality of transmission gears (52, 53, 54, 55, 56, 57, 58, 59) are engaged with each other; and a control unit that controls a rotational position of the stepping motor in accordance with an adjustment command by an occupant of the vehicle, characterized in that: at least one of the plurality of transmission gears (52, 53, 54, 55, 56, 57, 58, 59) that is predicted to incur a creep deformation is defined as a specific gear; an environmental temperature of the specific gear, at which the specific gear is predicted to incur the creep deformation, is defined as a predicted deformation temperature (Te); a deviation predicted to be caused at the virtual image display position by the creep deformation at the predicted deformation temperature (Te) is defined as a predicted display deviation (δDe); and the control unit corrects the rotational position in a direction to reduce the predicted display deviation (δDe) when the environmental temperature rises to the predicted deformation temperature (Te) or higher. at least one of the plurality of transmission gears (52, 53, 54, 55, 56, 57, 58, 59) that is predicted to incur a creep deformation is defined as a specific gear; an environmental temperature of the specific gear, at which the specific gear is predicted to incur the creep deformation, is defined as a predicted deformation temperature (Te); a deviation predicted to be caused at the virtual image display position by the creep deformation at the predicted deformation temperature (Te) is defined as a predicted display deviation (δDe); and the control unit corrects the rotational position in a direction to reduce the predicted display deviation (δDe) when the environmental temperature rises to the predicted deformation temperature (Te) or higher.