CCD1 as an image reading part reads a copy at the upstream and downstream of reading positions, and image data B and A at each of the reading positions are obtained from an output delay circuit and a shading correction circuit, respectively. A streak detector 8 compares the image data A with B, and outputs black streak detecting data indicating that the image data A contains black streaky noises, when the image data is not coincident in succession as to a plurality of pixels arrayed in the sub-scanning direction. A streak eliminator normally outputs the image data A, but it outputs the image data B instead of the image data A with regard to the pixels from which the black streak detecting data containing the noise is outputted. The image data reading device of present invention being thus constructed, the device is able to accurately detect streaky noises by adhesion of thrashes to the reading unit, even if there are speed variations in the copy feeding.