A calibrating apparatus for an image processing apparatus is disclosed. The calibrating apparatus comprises a first operating module and a second operating module. The first operating module determines a third indicating point and a fourth indicating point according to a first indicating point, a second indicating point, and a specific point, and determines a calibration point (i.e. the predetermined position of a sensor) according to a first line through the first indicating point and third indicating point and a second line through the second indicating point and fourth indicating point. The second operating module forms reference lines according to reference points and the calibration point, forms reference angles according to the reference lines and a parallel line, generates reference coordinates of the reference points in an image and generates a calibration function according to the reference coordinates of the reference points and the reference angles.