An offset amount calibrating method that obtains the offset amount between a contact-type detector and an image probe is provided. The method includes: setting on a stage a calibration chart that includes not less than two non-parallel linewidth patterns being disposed relative to a reference position of the calibration chart and each having a known width and a level difference; capturing an image of the linewidth patterns of the calibration chart by an image probe to obtain the reference position of the calibration chart; measuring at least two of the linewidth patterns of the calibration chart by a contact-type detector to obtain the reference position of the calibration chart; and calculating a difference between the reference position obtained by using the image probe and the reference position obtained by using the contact-type detector to obtain the offset amount.