A headup display apparatus consists of a projection optical system unit installed in the dashboard of a car and a reflection member on the windshield. The projection optical system unit contains an indicator and an off-axis reflective hologram which reflects display light of the indicator toward the windshield, which in turn reflects it toward the viewing point of a driver. As a result, the display is seen superimposed on the exterior view. When external light as from the sun enters the projection optical system unit traveling in a direction reverse to the path of the display light, the off-axis reflective hologram reflects the incident external light toward a direction different from that of the outgoing display light to minimize an abnormal illumination of the indicator surface which would otherwise render the display unrecognizable. Behind the hologram is installed a heat dissipating member that receives infrared rays that have transmitted the hologram in order to dissipate heat.