To provide a compact apparatus by simplifying the structure of a BD sensor and the optical path setting to the BD sensor, and reducing the number of optical components, such as a mirror for setting the optical path by commonly using the components.

Respective loopback mirrors 210, 208 and 205 for Y, M and C have a reflecting region 217 and a transmitting region 216, reflect light beams in an image region toward the optical path directed to a photoreceptor drum, and pass synchronization detection beams in a non-image region directed toward the optical path directed to the BD sensor. A mirror for K 204 reflects a light beam for K, and the synchronization detection beam for K in the non-image region is focused on the BD sensor, together with the synchronization detection beams which pass through the loopback mirrors 210, 208 and 205.