To easily and automatically align the optical axes of an illumination slit plate and measurement slit plate of a densitomer.

In a densitomer having an illumination optical system for applying measurement light 10 to a recording medium 16, an illumination slit plate 14 with a fine slit 14a for passing the measurement light 10 toward the record medium 16, a measurement slit plate 23 with a fine slit 23a for passing light through the record medium 16, and a photodetector 25 for detecting light through the measurement slit plate 23, the measurement slit plate 23 is moved in a direction for crossing a light axis by traveling means 31, 32, 33, and 35, and the measurement plate 23 is arranged at a position, where the quantity of received light of the photodetector 25 is maximized by a control means 34, based on the output of the photodetector 25 for each traveling position of the measurement slit plate 23.