A defect observation device including an input-output unit supplied with information of a taught defect, and information of an ideal output of the taught defect, and configured to display a processing result based upon a determined image processing parameter set; and an automatic determination unit configured to: select image processing parameter sets which are less in number than the total number of all image processing parameter sets, out of all image processing parameter sets, calculate image processing results on an input defect image, by using the selected image processing parameter sets, calculate a coincidence degree for each of the selected image processing parameter sets, estimate distribution of an index value in all image processing parameter sets from distribution of the coincidence degree for the selected image processing parameter sets, and determine an image processing parameter set to have a high coincidence degree out of all image processing parameter sets.