1. A visual inspection method for performing a visual inspection of semiconductor devices using a computer system provided with an image detector unit, an image processor unit and a display unit, the visual inspection method comprising: a classification condition setting step of setting classification conditions beforehand; a defect detection step of detecting defects by using inspection images acquired by imaging a target substrate, via the image detector unit, and calculating features of the defects; and a defect classification step of classifying the defects in accordance with the classification conditions set beforehand in the classification condition setting step using the features of the defects calculated in the defect detection step, via the image processor unit; wherein said classification condition setting step further comprises: a collection step of detecting a large number of defects by using images acquired by imaging a sample substrate, which are different from the inspection images acquired at the defect detection step, calculating features of each defect over the large number of defects, and collecting calculated features of each defect over the large number of defects to store collected features of each defect; a defect sampling step of creating a defect feature distribution indicating a defect occurrence distribution based on the collected features of each defect over the large number of defects collected in the collection step, and performing sampling of review defects based on the defect feature distribution; and a reviewing step of providing at least defect classes to a plurality of review defects by reviewing the review defects sampled in the defect sampling step; wherein the defect classes of the review defects provided in the reviewing step for features of the review defects collected in the collection step, are set as training data of the classification conditions.