A method and apparatus for detecting a defect in which image signals of a same area of a sample are obtained by imaging the sample under different optical conditions, and the obtained image signals are analyzed and optical conditions are selected which modify a contrast in the image signal. Image signals of the sample under the selected optical conditions are obtained by imaging the sample with an inspection system, and the images under the selected optical conditions are evaluated to adjust optical conditions for inspection including an inspection threshold, which is greater than a maximum contrast difference among false defects detected at the step of obtaining and with which a maximum number of actual defects can be detected. A defect of the sample is detected by processing the image signals of the sample obtained through the inspection system under the adjusted optical conditions.