A distance between a reference position and a reference image of a sheet is accurately measured without adding a special device even when an attitude of the sheet is not stable. An aspect of the present invention obtains image data by reading a sheet surface formed with the reference image such as a trim mark by a reading portion. A height calculation portion calculates edge-blur amounts of the reference position and the reference image on the sheet surface from the image data. Furthermore, a height calculation portion detects heights from a focus position to the sheet surface for the reference position and the reference image, from the edge-blur amount and a reflected light amount of a white solid portion. Moreover, a distance detection portion calculates a distance between the reference position and the reference image on the sheet surface, on the basis of a detection result.