An image-forming apparatus includes an optical sensor including a light-receiving element that receives light reflected by a belt-like moving medium to produce an output corresponding to the amount of light received. This image-forming apparatus determines the density of a toner patch image formed by an image-forming unit by correcting an output produced when the optical sensor detects the toner patch image at a position on a roller disposed opposite the sensor on the basis of an output produced by the optical sensor in an area where no toner image is formed on the belt-like moving medium at the same position on the opposite roller as the position where the sensor detects the toner patch image.