PURPOSE: To increase an inspection efficiency by switching over a threshold level in accordance with a spot of inspection of a wiring pattern and thereby conducting determination of a defect.

CONSTITUTION: An image signal of a wiring pattern on a printed circuit board 5 is subjected to A/D conversion 82 and turned to be binary data by each of binary-coding circuits 83 and 84 having high and low threshold levels respectively. By switching over the threshold level by a binary-coded image switching element 85, each inspection level in a bonding pad part and an ordinary wiring part of the board 5 is switched over. Accordingly, determination of a defect in accordance with a spot of inspection can be conducted. This switching can be executed in the course of scanning by a camera 81.