A head-up display device comprising: a liquid crystal display device configured to have a liquid crystal panel and a backlight to illuminate the liquid crystal panel from back side, and to illuminate transparent display illuminated by the backlight; a control means configured to drive control the liquid crystal display device; and an optical system configured to reflect a display light that is emitted from the liquid crystal display device while the liquid crystal display device transparently displays a virtual image, the optical system being further configured to reflect the display light at least one time toward a transparent member such as a combiner or a windshield to display the virtual image; wherein the optical system consists of at least one reflecting member (21,22) and the reflecting member (21,22) is a transparently reflecting member (21,22) which a certain wavelength of light passes through and reflects the light except the certain wavelength of light, and the head-up display device further comprising a detection means, placed behind of the transparently reflecting member (21,22), for detecting an intensity of natural light upon receiving the certain wavelength of light which is transmitted in reverse direction of a pathway of the display light between the liquid crystal display device and a transparent member, and the certain wavelength of light passes through the transparently reflecting member (21,22), and the control means is configured to control the backlight to decrease the brightness of the backlight or turn off the backlight upon the intensity detected by the detection means overcomes the predetermined threshold. a liquid crystal display device configured to have a liquid crystal panel and a backlight to illuminate the liquid crystal panel from back side, and to illuminate transparent display illuminated by the backlight; a control means configured to drive control the liquid crystal display device; and an optical system configured to reflect a display light that is emitted from the liquid crystal display device while the liquid crystal display device transparently displays a virtual image, the optical system being further configured to reflect the display light at least one time toward a transparent member such as a combiner or a windshield to display the virtual image; wherein the optical system consists of at least one reflecting member (21,22) and the reflecting member (21,22) is a transparently reflecting member (21,22) which a certain wavelength of light passes through and reflects the light except the certain wavelength of light, the optical system consists of at least one reflecting member (21,22) and the reflecting member (21,22) is a transparently reflecting member (21,22) which a certain wavelength of light passes through and reflects the light except the certain wavelength of light, and the head-up display device further comprising a detection means, placed behind of the transparently reflecting member (21,22), for detecting an intensity of natural light upon receiving the certain wavelength of light which is transmitted in reverse direction of a pathway of the display light between the liquid crystal display device and a transparent member, and the certain wavelength of light passes through the transparently reflecting member (21,22), and the control means is configured to control the backlight to decrease the brightness of the backlight or turn off the backlight upon the intensity detected by the detection means overcomes the predetermined threshold.