1. A computer-aided inspection system for inspecting the external appearance of circuit substrates during a manufacturing process, and for displaying an abnormality grade and a presumed cause of an abnormality occurring during the manufacturing process, comprising: defect detection means for producing two-dimensional image signals, each representing the external appearance of one of said circuit substrates, and for detecting characteristics, including coordinates of defect candidates for each of said circuit substrates by comparing each of said two-dimensional image signals with a reference signal; first storage means for storing said detected characteristics and coordinates of defect candidates over said plurality of circuit substrates manufactured by said manufacturing process; actual defect extraction means for extracting actual defects from said defect candidates by grouping defect candidates having coordinates within a predetermined distance range from coordinates of candidates of defects stored in said first storage means for each of said plurality of circuit substrates manufactured by said manufacturing process, and for calculating locations for each of said extracted actual defects; an actual defect presumption knowledge base for storing a presumption rule of kind of actual defect, the presumption rule being based on a plurality of correspondencies between a combination of said characteristics and actual kinds of factors of said actual defects; presumption means for presuming a kind of factor for generating an actual defect from a combination of said stored characteristics of defect candidates stored in said first storage means, in accordance with said presumption rule; second storage means for storing said calculated locations for each extracted actual defect extracted by said actual defect extraction means and for storing said presumed kind of factor for each actual defect presumed by said presumption means; feature parameter calculation means for calculating a two-dimensional distribution of actual defects for said each kind of factor in at least one of said circuit substrates from said locations for actual defects stored in said second storage means, and for calculating a variation of the number of said actual defects for each kind of factor stored in said second storage means over said plurality of circuit substrates manufactured by said manufacturing process; an abnormality judgment knowledge base for storing an abnormality judgment rule, the abnormality judgment rule being based on a two-dimensional distribution reference and a reference number variation for each kind of factor; abnormality judgment means for determining an abnormality grade according to a partiality of a two-dimensional distribution obtained by comparing said calculated two-dimensional distribution for said each kind of factor with said two-dimensional distribution reference for each kind of factor, and according to an increase rate of number variation of actual defects obtained by comparing said calculated number variation of said actual defects for each kind of factor with said reference number variation for each kind of factor in accordance with said abnormality judgment rule stored in said abnormality judgment knowledge base; a cause-of-abnormality presumption knowledge base for storing a presumption rule of cause of abnormality based on (1) a plurality of correspondencies between a combination of the two-dimensional distribution of said actual defects and the number variation of said actual defects for said each kind of factor, and (2) causes of abnormalities; cause-of-abnormality presumption means for presuming a cause of the abnormality of said manufacturing process from a combination of said calculated two-dimensional distribution of actual defects and said calculated number variation of said actual defects for said each kind of factor stored in said second storage means, in accordance with said presumption rule of cause of abnormality; and display means for displaying said abnormality grade determined by said abnormality judgment means and said cause of abnormality of said manufacturing process presumed by said presumption means.