Abstract: A slip has a sample area which has a print of the drop-out colors used for the slip. A facsimile apparatus receives the image of the slip from a different facsimile apparatus. The facsimile apparatus produces a printed paper sheet, which is input to a character reader for photoelectric conversion into image data. Image data is stored in a memory. An image of the sample area is transferred to the area buffer. The detector detects the number of black dots in the dots constituting the image of the sample area stored in the area buffer. The detector compares the detected black dot number to a reference value. The detector determines that the drop-out colors have failed to be dropped out when the black dot number is greater than the reference value. The detector determines that the drop-out color was dropped out when the black dot number is smaller than the reference value.