1. A defect observation method for observing a defect of a sample, comprising the steps of: picking up a plurality of defect images by using an electron microscope on the basis of previously detected defect coordinates of a sample; processing a teaching defect image selected out of the plurality of picked up defect images, by using respective conditions of a first plurality of image processing parameter sets which are previously set, and extracting a plurality of defect areas respectively corresponding to the first plurality of image processing parameter sets; comparing an ideal defect area which is set to correspond to the selected teaching defect image with the extracted plurality of defect areas and calculating a coincidence degree every the plurality of defect areas; calculating an estimated value of coincidence degree with respect to each of a second plurality of image processing parameter sets different from the first plurality of image processing parameter sets which are previously set, by using the coincidence degree calculated for each of the plurality of defect areas; selecting one or a plurality of image processing parameter sets out of the first plurality of image processing parameter sets which are previously set and the second plurality of image processing parameter sets on the basis of the calculated plurality of coincidence degrees and estimated values of coincidence degree; and conducting image processing on the plurality of defect images picked up with the electron microscope by using the selected image processing parameter set and classifying the defect of the sample on the basis of the defect images subjected to the image processing.