A workpiece inspection apparatus includes a search unit for finding from an input reference image a first pixel group which contains a certain pixel and a second pixel group having grayscale values within a threshold, and a probability acquisition unit responsive to receipt of an optical image of a workpiece being tested and the reference image, for obtaining a probability value based on a grayscale value of the certain pixel, a grayscale value of its corresponding pixel in the second pixel group, a grayscale value of an optical image pixel corresponding to the certain pixel, and a grayscale value of an optical image pixel corresponding to a pixel in the second pixel group corresponding to the certain pixel. The probability value is used to determine whether a defect exists at the optical image pixel corresponding to the certain pixel.