Abstract: An image reading device comprising a light source for illuminating a medium to be read and a line sensor including a line of photoelectric conversion elements for receiving light from the medium and producing an image signal. White levels are determined for respective intervals into which the line of photoelectric conversion elements are divided. The device sets threshold levels for the respective intervals on the basis of the white levels for the respective intervals, and the image signal is compared with the set threshold levels to produce a series of binary signals. The condition of the light source being "off" not lit is determined on the basis of the relative magnitude between the white levels for the intervals.