An image forming apparatus which is capable of detecting the direction and amount of an image shift in the main scanning direction without wasting toner, to thereby provide a higher-quality image with reduced running costs. A conductor is disposed such that the conductor partially overlaps an electrostatic latent image line formed on a photosensitive drum in a manner extending in a main scanning direction of the photosensitive drum, while moving relative to the electrostatic latent image line. The conductor generates induced current by the relative motion. An image shift in the main scanning direction is detected based on a result of measurement of the induced current generated by the conductor.