An image quality detecting apparatus detects image quality based on a specified image pattern formed on an image carrier. This apparatus includes a light-emitting device that radiates a spotlight on the image carrier, a lens, a scanning unit that scans the image pattern with the spotlight, and a photoelectric conversion element that detects a quantity of light reflected from the image pattern and the image carrier or light transmitted through the image pattern and the image carrier during the scanning. The image quality is detected by setting a diameter of the spotlight at least in a scanning direction to the reciprocal number of a spatial frequency or smaller in which human eyesight is the most sensitive, for example, to 1000 .mu.m or less.