To measure color registration slippage stably with high accuracy.

A test pattern 33 of at least several dot width per line is printed in the main scanning direction and subscanning direction while developing each thermal coloring layer with maximum density. The test pattern 33 is read out by means of a scanner 31, and an arithmetic unit 32 measures the luminance of three colors RGB for each dot in the line width direction of the test pattern 33 thus read out. Average luminance in the line width direction is then calculated from the measurements and the luminance ratio of the three colors RGB is measured for each dot from the average luminance thus calculated. A color generating color registration is specified from the calculation results of luminance ratio and its direction and amount of slippage are calculated.