1. A 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, and calculating features of the detected defects; and a defect classification step of classifying the defects in accordance with the classification conditions set beforehand in the classification condition setting step by using the features of the defects calculated in the detection step,; wherein said classification condition setting step further includes: a collection step of detecting a large number of defects by using inspection images acquired by imaging a target substrate, and calculating feature of each defect over the large number of detected defects, and collecting the calculated feature of each defect over the large number of detected defects to store the collected feature of the each defect; a defect sampling step of creating defect feature distribution indicating defect occurrence distribution based on the feature of each defect over the large number of defects collected in the collection step, and performing sampling review defects based on the created defect feature distribution; and a reviewing step of giving at least defect classes to a plurality of review defects by reviewing for the review defects sampled in the defect sampling step; wherein the defect classes of the review defects given in the reviewing step for the features of the review defects collected in the collection step, are set as training data of the classification conditions.