1. A flaw detection apparatus for detecting a flaw formed on an object's surface, based on first digital image data indicative of said object's surface, as a first group of disconnected images adjacent to each other and having substantially the same directional orientation, said apparatus comprising: a first direction detection means for detecting a direction in which said first group extends based on a contour thereof; a first cluster means for combining a plurality of images of said first group that are disconnected and extend in the same direction; a first flaw signal producing means which calculates an area of said combined first group and produces, when the area of said combined first group exceeds a predetermined value, a first flaw signal indicating that said combined first group represents a flaw; said first flaw signal producing means includes: a first flaw coefficient determining means for calculating an area of said combined first group to determine a first flaw coefficient; and a second flaw coefficient determining means for calculating an area of rectangular region circumscribing said combined first group to determine a second flaw coefficient.