Apparatus for detecting defect

An apparatus for detecting a defect comprises a specimen, an illuminating section for irradiating the specimen with light disposed while inclining at a specified angle to the specimen, an image pickup section for picking up diffracted light from the specimen disposed while inclining at a specified angle to the specimen, an angle control section for varying the inclination angle of at least one of the illuminating section and the image pickup section, an image processing section for capturing the diffracted light from the specimen through the image pickup section for each inclination angle and determining a luminance value corresponding to the inclination angle of the diffracted light, a section for recognizing an n-order light most suitable for observation of the diffracted light from the relation between the inclination angle and the luminance value, and an angle control section for setting the inclination angle of at least one of the illuminating section and the image pickup section based on the result of the recognition.