PURPOSE: To improve the deciding accuracy for a defect in a pattern by comparing a light quantity difference of transmission light beams of two same patterns with a reference value and inspecting the defect of the pattern.

CONSTITUTION: When it is assumed that the light quantity which as transmitted through a pattern 3A is more than the light quantity which has transmitted through a pattern 3B, a level of an output signal of a pre-amplifier 6B becomes higher than a level of an output signal of a pre-amplifier 6A. Its output is inputted to non-inversion and inversion input terminals, respectively of a differential amplifier 7'. Subsequently, a positive voltage being proportional to a difference of the transmission light quantities of the pattern is outputted. As a result, an NPN transistor (TR) TB becomes ON and an electric lamp LB is lighted. On the other hand, a TR TA is OFF and an electric lamp LA is not lighted. When the elcetric lamp LB is lighted, the gain of an operational amplifier 8B drops, and corrected so that a difference of the transmission quantities of light beams to the pattern 3A and 3B decreases. An output of the amplifier 8B is compared with an output of an amplifier 8A by a comparing circuit 9, and when a difference of both the outputs exceeds a reference value, it is decided that there is a defect, and a signal of a high level is outputted from the circuit 9.