An improved image detection apparatus is described which can colorimetrically measure an appropriate number of patches while inhibiting unnecessary sheet discharging. A correction chart consists of a plurality of patches linearly arranged in the form of a plurality of patch arrays which are parallelly arranged in the sheet width direction. The image detection apparatus includes a paper conveying unit for conveying a sheet on which the correction chart is formed, an image reading unit and a colorimetric unit which outputs the color information of each patch by performing the colorimetric process with the patch array consisting of a line of the patche. Then, while circulating a sheet, a control unit performs switch control for switching the patch array from one to the next to be colorimetrically measured by the colorimetric unit.