1. An image reading device, comprising: a plurality of image sensors configured to read image data of an original document, each of which having a reading range overlapping in a main scanning direction with an adjacent image sensor from among the plurality of image sensors; and a gamma correction mechanism configured to conduct gamma correction to correct linearity of respective output characteristics of the plurality of image sensors, based on gamma correction data produced according to an output characteristic of a reference image sensor selected from the plurality of image sensors.