1. An image forming apparatus comprising: a scanning unit which optically scans an image of an original; a process unit which includes image bearing members and exposes the image bearing members according to the image scanned by the scanning unit to thereby print the scanned image on a paper sheet; a storing unit having stored therein a test pattern image; a control section which controls the process unit to print the test pattern image stored in the storing unit on the paper sheet; a detecting section which detects, when the printed test pattern image is scanned by the scanning unit, a difference between density of the scanned test pattern image and absolute density set in advance; and an adjusting section which adjusts an exposure amount of the process unit according to the density difference detected by the detecting section.