PURPOSE: To certainly interpret the existence of defects in an opaque matter with a printing region or the like by subtracting a test region image from a reference region image as a criterion for ink shortage in the test region and subtracting the reference region image from the test region image as a criterion for an unnecessary ink part.

CONSTITUTION: An output from a camera 70 is sent sequentially to a monitor 85 and a frame grabber 86. In addition, the frame grabber 86 is made to communicate with a computer 88 and at the same time, these components are made to communicate with a processor 90. Further, data is given and received between a port 92 and the computer 88. On the other hand, a signal 96 from a conveying device is sent to the port 92 through an interface 94. After that, a refusal signal 98 is output from the computer 88 through the port 92 and the interface 94. In this case, a test region image is subtracted from a reference region image as a criterion for ink shortage in the test region and the reference region image is subtracted from the test region image as a criterion for an unnecessary ink part.