To perform sufficient density correction considering its change over aging in an image forming device.

The pattern densities of plural gradations formed by a printer is measured previously by a scanner to prepare a density correction table for correcting the density to have prescribed characteristic, and the density of a pattern formed inside the printer is measured with a sensor to set its reference. After then, with prescribed timing, the density of the pattern formed in the printer is measured with the sensor (S1404) and based on a density variation obtained by comparing the measured value with the reference value (S1402), a density correction table is corrected (S1403). Thus, it becomes possible to perform density correction considering its change over aging in the printer.