1. An image-forming apparatus comprising: an image-forming unit that forms an image on a sheet being transported; and an image-reading unit that is disposed downstream of the image-forming unit in a transport direction of the sheet and that reads the image formed on the sheet being transported, wherein the image-forming unit has a test mode in which a test image is formed on a sheet, the test image including a defect detection image for image defect detection and a plurality of marks for detecting the angle of inclination of the test image on the sheet in a two-dimensional plane extending in the transport direction and a width direction of the sheet crossing the transport direction, the plurality of marks being formed at positions remote from each other in a central region of the sheet in the width direction, and wherein the image-reading unit reads the test image to acquire test image data representing the test image, the image-forming apparatus further comprising: a computing unit that performs an image defect detection operation based on the test image data, the computing unit including: a mark-detecting section that detects the plurality of marks; an inclination-calculating section that calculates the angle of inclination of the test image in the two-dimensional plane based on the plurality of marks detected by the mark-detecting section; an inclination-correcting section that corrects the test image for inclination based on the angle of inclination calculated by the inclination-calculating section to generate corrected-image data representing the corrected test image; and a defect-detecting section that detects an image defect based on the corrected-image data generated by the inclination-correcting section.