A vehicle display device (1, 1A, 1B, 1C) comprising: a housing comprising an internal space and an opening, the opening communicating between an outside of the vehicle display device and the internal space; the housing houses a display, at least one reflective mirror (3, 11, 13), a controller, an optical sensor, a drive unit, a reflective optical member in the internal space and supports them, the housing further housing an inner housing which supports at least the optical sensor and the reflective optical member; the display emits from the housing via the opening, as display light, a display image projected on a projection target provided outside the vehicle display device (1, 1A, 1B, 1C); the at least one reflective mirror (3, 11, 13) is disposed on a light path of the display light from the display to the projection target and reflects the display light; the reflective optical member is on a light path of the display light between the reflective mirror (3, 11, 13) and the display and disposed at a position facing the display, transmits the display light emitted from the display toward the reflective mirror (3, 11, 13), and diffusion reflects external light directed from the reflective mirror (3, 11, 13) toward the optical sensor; the optical sensor is configured to receive light in the infrared region of the external light to convert the light into an electric signal and output the electric signal to the controller as a detection signal, and detects the external light entering from the opening communicating between the outside of the vehicle display device (1, 1A, 1B, 1C) and the internal space; and the controller is configured to execute temperature decreasing operation for reducing a temperature of the display when the optical sensor detects the external light, and the optical sensor is disposed at a position to which the external light directed toward the display via the reflective mirror (3, 11, 13) is diffusion reflected by the reflective optical member, the reflective optical member is disposed in a direction orthogonal to an emission direction of the display light emitted from the display toward the reflective mirror (3, 11, 13), and the inner housing is formed to secure a light path of display light from the reflective optical member to the projection target and to secure the light path of the external light from the reflective optical member to the optical sensor. a housing comprising an internal space and an opening, the opening communicating between an outside of the vehicle display device and the internal space; the housing houses a display, at least one reflective mirror (3, 11, 13), a controller, an optical sensor, a drive unit, a reflective optical member in the internal space and supports them, the housing further housing an inner housing which supports at least the optical sensor and the reflective optical member; the display emits from the housing via the opening, as display light, a display image projected on a projection target provided outside the vehicle display device (1, 1A, 1B, 1C); the at least one reflective mirror (3, 11, 13) is disposed on a light path of the display light from the display to the projection target and reflects the display light; the reflective optical member is on a light path of the display light between the reflective mirror (3, 11, 13) and the display and disposed at a position facing the display, transmits the display light emitted from the display toward the reflective mirror (3, 11, 13), and diffusion reflects external light directed from the reflective mirror (3, 11, 13) toward the optical sensor; the optical sensor is configured to receive light in the infrared region of the external light to convert the light into an electric signal and output the electric signal to the controller as a detection signal, and detects the external light entering from the opening communicating between the outside of the vehicle display device (1, 1A, 1B, 1C) and the internal space; and the controller is configured to execute temperature decreasing operation for reducing a temperature of the display when the optical sensor detects the external light, and the optical sensor is disposed at a position to which the external light directed toward the display via the reflective mirror (3, 11, 13) is diffusion reflected by the reflective optical member, the reflective optical member is disposed in a direction orthogonal to an emission direction of the display light emitted from the display toward the reflective mirror (3, 11, 13), and the inner housing is formed to secure a light path of display light from the reflective optical member to the projection target and to secure the light path of the external light from the reflective optical member to the optical sensor.