1. A pattern measurement apparatus comprising: an irradiation unit for irradiating a sample with an electron beam; an electron detection unit for detecting an amount of electrons generated, by the electron beam radiation, from the sample on which a pattern is formed; an image processor for generating an SEM image of the pattern on the basis of the amount of electrons; and a controller for acquiring a rectangular measurement specification region specifying a measurement target portion of the SEM image, and then calculating a loss ratio of a corner portion of the pattern from an area of the measurement specification region and an area of the corner portion of the pattern, wherein the controller sets a position of the rectangular measurement specification region so that a pair of corners arranged on an oblique of the rectangular measurement specific region contains a corner portion of the pattern, and then sets the corner of the rectangular measurement specific region coincides with a side of the pattern of the SEM image; wherein the controller detects an average value of edge positions in a predetermined range of the pattern of the SEM image, where the predetermined range includes a position that the corner of the rectangular measurement specification region and the side of the pattern of the SEM image intersect with each other; and wherein the controller moves the rectangular measurement specification region so that the corners of the rectangular measurement specification region coincide with the edge positions corresponding to an average value of side positions of the pattern of the SEM image.