PURPOSE: To measure the density of a mark recorded on a medium, by providing a differential amplifier for outputting a difference between an output of an integral network and an output of a photodetector, and a comparison circuit for comparing an output of the differential amplifier with the reference.

CONSTITUTION: A density detection mark 2 printed at a specific position of a form 1 is irradiated by a light beam from a light emission source 3. Its refracted ray is detected by a photodetecting amplifier 4, also is implified, and is outputted. Its output voltage is applied to an integral network 7, is smoothed and is supplied to one input terminal of a differential amplifier 8. On the other hand, output voltage of the photodetecting amplifier 4 is supplied to the other input terminal of the differential amplifier 8. The differential amplifier 8 outputs a difference between an output of the integration network and an output of the photodetecting amplifier 4. Its output is compared with constant voltage of a constant voltage circuit 5 by a comparing circuit 6, and as a result, the density of a mark is measured exactly without being influenced by a material quality of the form, or mechanical variations of a detection mechanism, by constituting so as to detect density of a density detection mark 2 which exceeds a reference value out of them.