A defect inspection apparatus includes a sensor which optically senses a circuit pattern formed on a plate to be inspected to obtain scanned image data thereof while moving relatively to the plate, an AD converter which converts the scanned image data into digital form, a normal image data generator which generates normal image data expressed by use of multiple values based on CAD data relating to the circuit pattern, a reference data generator which filters the normal image data to generate reference data while selecting filter coefficients according to the moving direction of the plate to be inspected by use of a finite response filter having asymmetrical coefficients, and a comparator which compares the reference data with the scanned image data.