An array light source 1 with semiconductor laser sources disposed one-dimensionally and a projective lens 2 are used to illuminate an inspected object so that light beams projected from the array light source form a dotted line on the object. A line sensor is used to receive through an objective lens 3 light emitted from an imaging area 11 away from an illuminated area 12. An image signal, fed to an image processing unit 8 through a pre-processing unit 7 producing an image from signals from the line sensor 4 and a stage 5 is processed, while the stage 5 bearing the object 6 is being gradually moved, to inspect the object 6 for crack defects 9 and 10 by detecting an optically nonhomogeneous portion of the object. The method allows a crack defect of an object, such as a ceramic substrate or a sintered metal product, to be detected fast with high accuracy.