A cam member has each of a plurality of first exposure-side surfaces provided in association with a first contact part of each exposure units and each of a plurality of second exposure-side surfaces provided in association with each first contact part. Each first contact part confronts but separates from the corresponding first exposure-side surface when the corresponding exposure unit is disposed in an adjacent position where the exposure unit is capable of exposing the corresponding photosensitive drum. Each first contact part is in contact with the corresponding second exposure-side surface when the corresponding exposure unit is disposed in a retracted position positioned farther than the adjacent position from the photosensitive drum.