A vehicle display device comprising: a display device configured to emit, as display light, a display image to be projected onto a projected member disposed outside the vehicle display device; at least one reflecting mirror (3, 11, 13) disposed on an optical path of the display light from the display device to the projected member and configured to reflect the display light; an optical sensor configured to detect external light that enters the display device through an opening allowing the outside and an internal space to communicate with each other and through the reflecting mirror (3, 11, 13); and a controller configured to enable execution of a temperature lowering operation for lowering a temperature of the display device when the optical sensor detects the external light, wherein the display device includes: a light source; a light transmissive liquid crystal display element disposed on an optical path of light emitted from the light source; and is characterized in that it further includes a first optical member disposed between the liquid crystal display element and the light source and configured to change an optical path of light that enters from the light source and passes through the first optical member toward the liquid crystal display element, the optical sensor is disposed on an optical path of external light incident on the display device at a side opposite to the liquid crystal display element across the first optical member, and the first optical member includes a passage on an optical path through which the external light passes. a display device configured to emit, as display light, a display image to be projected onto a projected member disposed outside the vehicle display device; at least one reflecting mirror (3, 11, 13) disposed on an optical path of the display light from the display device to the projected member and configured to reflect the display light; an optical sensor configured to detect external light that enters the display device through an opening allowing the outside and an internal space to communicate with each other and through the reflecting mirror (3, 11, 13); and a controller configured to enable execution of a temperature lowering operation for lowering a temperature of the display device when the optical sensor detects the external light, wherein the display device includes: a light source; a light transmissive liquid crystal display element disposed on an optical path of light emitted from the light source; and is characterized in that it further includes a first optical member disposed between the liquid crystal display element and the light source and configured to change an optical path of light that enters from the light source and passes through the first optical member toward the liquid crystal display element, the optical sensor is disposed on an optical path of external light incident on the display device at a side opposite to the liquid crystal display element across the first optical member, and the first optical member includes a passage on an optical path through which the external light passes. a light source; a light transmissive liquid crystal display element disposed on an optical path of light emitted from the light source; and is characterized in that it further includes a first optical member disposed between the liquid crystal display element and the light source and configured to change an optical path of light that enters from the light source and passes through the first optical member toward the liquid crystal display element, the optical sensor is disposed on an optical path of external light incident on the display device at a side opposite to the liquid crystal display element across the first optical member, and the first optical member includes a passage on an optical path through which the external light passes.