PURPOSE: To perform correctly the visual comparing check of a wafer by putting marks on the wafer by means of signal output for errors in a comparator which outputs differential signals out of image signals.

CONSTITUTION: A comparator 10 which compares image output signals given by video cameras 6 and 7 leaves a differential mode as it is and a control device 12 observes the whole region of chips by moving wafer chucks 1 and 2 in parallel on a pedestal 3 by means of control signals 13. When visually defective places are detected, the control device 12 gives control signals 15 to a laser marker 1 6 to put marks on a chip. If no defective places are found after checking the whole surface of its chip, the other chip is checked. In this case, this device makes the wafer chuck 1 move to return to the initial position of such a superior chip and then, the wafer chuck 2 operates independently of the chuck 1 to move to the same position that another superior chip holds. The visual check of a wafer is thus carried out correctly at great speed as well.