PURPOSE: To obtain a density detection apparatus receiving no influent of disturbance light and capable of correcting sensitivity at every measurement, by alternately measuring densities of a reference density reflective body and an object to be inspected by using monochromatic laser light.

CONSTITUTION: Beam B emitted from a laser beam source 1 is split into beam B1 for irradiating a reference reflecting body 7 and beam B2 for irradiating printed matter Pt by a splitter 3. These beams B1, B2 are respectively reflected by the reference reflective body 7 and the printed matter Pt and directed to a beam receiving part 4. Next, an optical system consisting of elements 1W5, M is moved to the direction crossing a paper surface at right angles in the Fig. A and, when a motor M is rotated in synchronous relation to the moving speed of said optical system, the reflected beam B1' from the reference reflective body 7 and reflected beams B2' from at printed reagions a1, a2...an of the printed matter Pt are alternately incident to the beam receiving part 4 through the aperture 5a of a rotary disc 5. The beam receiving part 4 receives the correction of sensitivity at every density measurement of the printed regions a1, a2...an by the reflected beam signal from the reference reflective body 7.