PURPOSE: To inspect surface flaws with good S/N by joining the detect signals which are optically detected to each other by virtual defect signals, erasing the joining signals of only the two end parts and selecting the signals of the defects longer than a prescribed length.

CONSTITUTION: Thresholds are provided to the sizes of picture in order to eliminate noises and surface defect signals A, B..., etc. are detected by detecting the reflected light. The signals A, B... are extended and joined to each other in their direction by virtual defect signals a1 and a2, b1, and b2.... If only the defect signals in both end parts are erased and the defect signals which are made longer than the prescribed value by the joining are discriminated as a surface defect signal, the spurious defect signals owing to surface ruggedness, etc. are eliminated, and the surface defects are inspected with good S/N.