A document scanning device such as a copier has defined within itself a point at which the brightness of an original document is detected and a separate point at which the document is scanned. The level of light exposure given to the document is controlled according to the detected brightness. The distance between the aforementioned two points is set with the delay time of an exposure control circuit taken into account. The delay time is set according to the fastest scanning speed of the device and is increased by means of a delay circuit if the scanning speed is deceased.