An image forming apparatus which is capable of, even when the density of an image in a color with low reflectivity has decreased, detecting a misregistration of this color with high accuracy. A first image of a first color and a second image of a second value having lower reflectivity than that of the first color are transferred to an image carrier. A first signal corresponds to reflected light from a first measurement image, a second signal corresponds to reflected light from a second measurement image, and a third signal corresponds to reflected light from the image carrier. A threshold value is set based on the second signal and the third signal. A color misregistration between the first and second images is corrected based on information on a positional relationship between the first and second images determined by comparing the first signal and the threshold value.