1. A defect detection method comprising: extracting a slope angle of a pattern, which is not zero, from an original image in which a plurality of the patterns are formed at periodic intervals; extracting a pattern period at which the patterns are separated from each other, by using the slope angle; shifting the original image by the pattern period in a direction perpendicular to the non-zero slope angle to form a shifted image; combining the original image and the shifted image; and detecting an aperiodic defect by removing an overlapping area from the original image and the shifted image that are combined, wherein a remaining non-overlapping area is detected as the aperiodic defect; classifying a type of the aperiodic defect based on a plurality of pre-stored defective types, and wherein the extracting of a pattern period comprises: analyzing a darkness and brightness change in a straight line passing in a direction of the non-zero slope angle to generate a first profile; analyzing a darkness and brightness change in a straight line passing in the direction perpendicular to the non-zero slope angle to generate a second profile; and calculating the pattern period by using the first and second profiles, and wherein in the second profile, an interval at which patterns are separated from each other matches a period of a periodically formed pattern.