To identify an ink jet printer quantatively and without destroying a printing sample.

Image data obtained by a CCD camera 8, etc., with light reflected or transmitted on/through at least a part of the halftone dot part of the printing surface of a sample printing sample are analyzed by a frequency analytic part 18. The correlation of the analyzed data is obtained by an arithmetic part 20 with respect to the image data of a printing surface having the same quality with that of the printing surface of the standard printing sample of contrast printer stored in an external storage device to judge whether the sample printing sample is the same as the standard printing sample to identify the difference of ink jet printers.