PURPOSE: To judge printing error when the computed value exceeds the measured value, by obtaining the difference between a reference value and the amount of light reflected from the surface of paper to be detected at each measuring point, and designating the part, in which error points are continued, an error block.

CONSTITUTION: A digital signal from an AD converter 13 is stored in a memory circuit 75 for each measuring point in accordance with the instruction of a memory controller 14. The content of a memory 16 is compared with the content of a memory 15 by a poor-level and poor-point detector 31. When the difference exceeds a specified poor-level or poor-point, the signal is detected as the poor- point. An operator 50 is provided in order to perform the computation of L+ S/P, where L is a peak value of an error block, S is the sum of the levels of the error block, and P is the number of error points in the error block. A part, in which the error points are continued is designated to be the error block. When the computed value exceeds the preset value, the error is judged.