1. A defect inspection device comprising: an illumination optical system for illuminating a sample with light; a detection optical system for detecting scattered light from the sample illuminated by the illumination optical system; and a processing unit for detecting a defect on the basis of the scattered light detected by the detection optical system, wherein the processing unit includes: a defect feature quantity calculation unit that calculates a feature quantity of each defect candidate extracted on the basis of the scattered light detected by the detection optical system; a defect candidate grouping unit that performs grouping of the defect candidates on the basis of the feature quantity calculated by the detect feature quantity calculation unit; a defect classification evaluation value calculation unit that calculates a defect classification evaluation value of the defect candidate on the basis of the feature quantity calculated by the defect feature quantity calculation unit; a defect classification evaluation value updating unit that updates, on the basis of an instruction, the evaluation value calculated by the defect classification evaluation value calculation unit; a defect classification threshold determination unit that determines, on the basis of the evaluation value updated by the defect classification evaluation value updating unit, a classification boundary as a threshold for classification of the defect candidates into defect types; and a defect detection unit that detects a defect using the threshold determined by the defect classification threshold determination unit.