An image reading apparatus and an image forming system are provided which can reduce the influence of magnification error, detect patches in correct positions, and save the sheets required for adjusting an image. The image reading apparatus makes use of an image scanner which reads patches printed on a sheet P, a spectral colorimeter which colorimetrically measures the patches read by the image scanner, and is provided with a timing compensation unit which compensates the colorimetric measurement timing with which colorimetric operation of the spectral colorimeter is triggered based on the positional information of the patches read by the image scanner, and an image correction unit which corrects the color information of the patches read by the image scanner based on the colorimetric values of the patches colorimetrically measured by the spectral colorimeter.