PURPOSE: To enable the decision of print density and the detection of the omissions printing at a constant slice level regardless of changes in blank paper and ink by obtaining the ratio of an inspection level to the reference level of the density difference of a print, comparing the ratio with a ratio set value previously set and deciding the density to be effective when the ratio is a specified value or less.

CONSTITUTION: When the density data of a nondefective print are imparted, a changeover switch SW is changed over to the side A, and an output from a subtraction circuit C4 is stored in a reference level memory C5. When the data of a print to be inspected are imparted, the changeover switch SW is changed over to the side B, the output from the subtraction circuit C4 is sent to an inspection level memory C6 while a ratio arithmetic circuit C7 is operated, and the ratio of the memory value of the inspection level memory C6 to the memory value of the reference level memory C5 is computed, and transmitted over an out-of-printing decision circuit C9. A set ratio is sent to the omission in printing decision circuit C9 from a ratio setting circuit C8, and whether defective density corresponds to the omission in printing is decided by the output from the ratio arithmetic circuit C7. Accordingly, nondefective or defective density can also be decided regarding any printing by a specified slice level.