A device for verifying the integrity of an item of data displayed on a display device controlled by a video controller, the video controller being connected to the display device by an appropriate connection and transmitting to it a video signal including an input interface which allows the device to be connected at a branch of the connection between the video controller and the display device, a reconstruction device which is capable, from the derived video signal, of reconstructing an image corresponding to the image displayed on the display device; an analysis device which is capable of extracting an item of reconstructed data from the reconstructed image, a comparison device which is capable of comparing the reconstructed data with a reference value of the item of data to be displayed and an alarm means which is capable of activating a malfunction alarm in accordance with the result at the output of the comparison device.