1. An image input apparatus for acquiring image data in accordance with an exposure amount by exposing an image of an object, comprising: a composite image generating unit that generates composite image data using a plurality of exposure amounts for a plurality of exposures; a detection unit that detects an exposure change amount at one pixel position in the composite image data, using the plurality of exposure amounts; a reference image generating unit that compares an exposure change amount detected at each pixel position by the detection unit with a preset threshold, and generates reference image data composed of data pieces for distinguishing a pixel position at which the exposure change amount exceeds the threshold from the other pixel positions; a verification data generating unit that generates verification data for determining whether or not alteration has been made from the composite image data obtained by the composite image generating unit and the reference image data obtained by the reference image generating unit; and an output unit that outputs the composite image data, the reference image data, and the verification data.