PURPOSE: To highly accurately attain the defect inspection of objects with various shapes by forming an approximate function approximated with the outline of each object from image data, and extracting an area corresponding to the prescribed condition of the approxi mate function to form a mask.

CONSTITUTION: A video camera 4 picks up the image of an electron gun cathode 1 and its image signal is converted into a digital signal by an A/D converter 5, and the digital image is stored in an image memory 11. A binarizing processing part 13 is driven by a command outputted from a main control part 12 to process image data. Then the main control part 12 generates an operation command to an outline approximating part 14 and a boundary point extracting point 15 receives the image data and detects respective edge parts of the upper surface of a holder 2, the surface of a disk and a tab. An outline approximating part 16 applies the equation of a circle to respective boundary points '+', finds out respective outlines approximated with the array of respective boundary points '+' by least square and sets up the borderlines of respective straight lines. The main control part 12 generates an operation command to a mask forming part 17, an area partitioning part differentiates the density levels of respective areas surrounded by respective boundaries and an area extracting part 19 makes these areas white level and makes the other part black level.