An image-reading device, in which the 1-bit signals a CCD line sensor has generated from the light applied from an illumination lamp and reflected from an original are shading-corrected, thereby forming image data. Before scanning the original, a black reference signal, a white reference signal at a level higher than that of the black reference level, and three intermediate reference signals are generated and stored into memories. During the scanning of the original, a selector selects two of the reference signals in accordance with the level of each signal output by the CCD line sensor, and the difference between the signal output by the sensor and the black reference signal is shading-corrected, by using the two reference signals selected by the selector. The signal output by the CCD line sensor can therefore made linearly proportional to the intensity of the input light, even if the CCD line sensor has no linear input-output characteristic or even if the photoelectric elements of the sensor have different input-output characteristics.