PURPOSE: To detect the defects of two or more equal patterns presented in the same masks from the difference of interfering colors of developing pattern of a sensitizer by superposing and exposing a plurality of patterns composed in one mask on the sole sensitizer surface.

CONSTITUTION: A plurality of the same shaped patterns 2, 3 composed in one mask are superposed and exposed on a sole sensitizer surface 4, and patterned. In this case, it is performed under the conditions that the sum of the exposure amounts of the same patterns is larger than that in which the remaining film ratio of the sensitizer becomes 0 and once exposure amount becomes less than that in which the remaining film ratio of the same sensitizer becomes 0. If a circuit pattern 2 or a circuit pattern 3 has a defect, the light interfering colors become different between a defective portion and a nondefective portion, thereby detecting the defect. Which of the circuit patterns 2, 3 has the defect can be discriminated from the film thickness difference of the remaining film patterns A, B, i.e., the light interfering colors by setting the exposure amounts of both at Ea≠Eb.