In case of reading a black-and-white original, the LEDs of R (red), G (green) and B (blue) colors are maintained at turn-on duty ratios same as those at the color image reading, and are turned on simultaneously or in succession, with the turn-on period reduced to , whereby the forward current of each of the R, G, and B LEDs is maintained same in the color image reading mode and in the black-and-white image reading mode. Thus, in case of reading the black-and-white original with the light source-switched color image sensor, the current adjusting means for adapting to such reading modes can be dispensed with in the light emitting elements for illuminating the original image, and the service life of the device can be extended with the simple configuration.