A vehicle-mountable display device includes an image display unit configured to be mounted to a position of an inner rearview mirror in a cabin of a vehicle, a driving unit for driving the image display unit, and an orientation controller for controlling the driving unit to control an orientation of the image display unit. The orientation controller is configured to turn the orientation of the image display unit into a rearview-mirror-mode orientation when the image display unit operates at a rearview mirror mode in which the half-mirror is used as a rearview mirror. The orientation controller is configured to turn the orientation of the image display unit into a display-mode orientation different from the rearview-mirror-mode orientation when the image display unit operates at a display mode in which the image display displays the captured image. A normal line of the half mirror at the display-mode orientation is closer to a driver than the normal line of the half mirror at the rearview-mirror-mode orientation. The vehicle-mountable display device improves visibility of the image display for a driver.