PURPOSE: To highly accurately detect the developing density of a detection mark with a detection unit, by making the developing density detection means constituted of the light emitting part and the light receiving part for light receiving the reflection light of the light emitted from the light emitting part, and adopting the white light source.

CONSTITUTION: The OD sensor 30 as an optical density detection sensor is constituted as a reflection type sensor. Namely, the OD sensor 30 is composed so that the light emitting part for emitting the detection light DL toward the surface of the photoreceptor drum 12, and the light receiving part for light receiving the reflection light RL of the detection light DL by the surface of the photoreceptor drum 12, is included therein. For instance, the light emitting diode (LED) may be good for the light emitting part, or the white light source, etc., substantially including all kinds of the visible spectral may be good. Then, the light receiving part is composed of for instance, the photodiode and CCD element, etc. Moreover, the light emitting part and the light receiving part of the OD sensor 30 are positioned on the passing position of the detection mark. Thus, the optical density detection of the detection mark by the OD sensor 30 can be performed with high accuracy and high sensitivity.