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, wherein: the adjacent image sensor is configured to act as a correction target image sensor, and the reference image sensor and the correction target image sensor read identical input data, and the gamma correction mechanism compares output data of the reference image sensor and output data of the correction target image sensor, obtains a difference between these output data, and adjusts the output characteristic of the correction target image sensor to the output characteristic of the reference image sensor.