An image forming apparatus has a first stacking unit for stacking regular sheets on which images are formed, an image forming unit for forming an image on a regular sheet fed from the first stacking unit, a second stacking unit for stacking the insertion sheets, a delivery unit for discharging the regular sheets from the image forming unit and the insertion sheets from the second stacking unit into a single sheet discharge section, and a paper thickness detection unit provided on a common carrier path through which the regular sheets carried from the image forming unit and the insertion sheets delivered from the second stacking unit are transported for detecting the thickness of the regular sheets and the insertion sheets.