An image reading device having plural image sensors and a reading controller is disclosed. The image reading device is for obtaining image data over a subject by joining sub-image data outputs read by the image sensors, each image sensor comprising: a signal amplifier; and an A/D converter for converting an analog value output from the image sensor to a digital value; whereby the reading controller adjusts an amplification factor of each signal amplifier so that a read value of a white board read by each image sensor becomes a predetermined value.