1. An inspection method for a photomask pattern applying a vector comparing method, comprising the steps of: (a) obtaining a pair of optical images, which are to be compared, from the photomask pattern; (b) converting the pair of optical images into two groups of electrical analog signals; (c) distributing the two groups of electrical analog signals into two groups of digital data, each of the two groups of digital data comprising a first number of matrix elements represented by datums having a value representing one of black, gray and white in accordance with high, middle, and low amplitudes, respectively, of a corresponding analog signal in one of the two groups of electrical analog signals; (d) separating two groups of separated data from the two groups of digital data, sequentially and respectively, each of the two groups of separated data having a second number of matrix elements, the second number being less than the first number of matrix elements in each of the two groups of digital data; (e) shifting the two groups of separated data respectively by a shift amount of at least one matrix element of each of the two groups of separated data in at least one of a latitudinal and longitudinal direction to provide at least two groups of shifted data; (f) synthesizing the two groups of separated data and the at least two groups of shifted data, respectively, to provide at least two groups of synthesized data; (g) generating vectors from arrangements of the at least two groups of synthesized data to provide at least two groups of vectors, each of the two groups of vectors comprising at least one of: a first vector from one of the datums representing black to an adjacent datum representing white, the first vector having a direction in one of latitudinal, longitudinal, and diagonal directions, a second vector, having one of the directions, from one of the datums representing black to an adjacent datum representing gray, and a third vector, having one of the directions, from one of the datums representing gray to an adjacent datum representing white, the first, second and third vectors having equal magnitude; (h) summing amounts of vectors in each of the directions for each groups of vectors; (i) comparing the amounts summed in step (h), respectively in each of the directions, for the at least two groups of vectors, to provide differences between the amounts; and (j) producing an output that indicates the photomask pattern has a defect when any one of the differences is non-zero.