To provide a monitoring device capable of recognizing existence of a monitoring camera by a suspicious person.

The monitoring device is provided with a sensor for detecting abnormality of the vehicle; a camera for photographing an image; and a means for outputting the camera image photographed by the camera to an on-vehicle display device mounted on the vehicle. The monitoring device is constituted such that when the sensor detects abnormality, the camera image is displayed on the on-vehicle display device. Since existence of the monitoring camera can be recognized by the suspicious person, it can contribute to prevention of evil deed.