To recognize an abnormality in an optical system of an image forming device.

A first density correction procedure 16-1 corrects detected density value characteristics to a gradient by using a prescribed arithmetic formula on the basis of a detected density value to the gradient of a prescribed test patter printed, a second density correction procedure 16-2 reprints a test pattern on the basis of the result of the correction, and normalizes the result of the reprinting by a reference density in percentage of a 100% gradient in reference gamma characteristics, a measured gamma characteristics computation procedure 16-3 obtains an integrated density value from a first gradient to a second gradient by computing the measured gamma characteristics, a both gamma characteristics comparison procedure 16-4 compares both the gamma characteristics with each other and an alarm generation procedure 16-5 generates a warning when a difference of a prescribed amount or larger is recognized between the gamma characteristics.