A monitor used in an automobile includes an image sensing module, a visual field adjustor, and a display. The image sensing module includes a lens module and an image sensor secured on an emitting light path of the lens module. The visual field adjustor includes a prism made from transparent materials and movably installed in an incident light path of the lens module. When the automobile reverses, the image sensing module cooperates with the visual field adjustor to capture optical image signals, and the display uses the optical image signals to create an image and displays the image.