To reduce the trouble that a display image becomes invisible, etc., by detecting the temperature variation of a hologram, provided for windshield glass, with ambient temperature by a detector which is provided nearby and controlling the wavelength of incident light with its signal.

The detector 1 which detects temperature and a control circuit 2 are arranged nearby the combiner 12 of the windshield glass. Further, a filter 7, formed of a multilayered film, which is provided between a display element 3 and a halogen lamp 4 is fitted on the rotary shaft of a stepping motor 18 and variable with respect to the angle. Then temperature dependency such as the diffraction wavelength, diffraction angle, etc., of the hologram is previously inputted and a signal regarding the temperature is received by the control circuit 2 from the detector 1 to vary the angle of rotation of the stepping motor 18, i.e., the tilt of the filter 7 and the voltage applied to the halogen lamp 4 so as to match diffraction characteristics of the hologram at this temperature.