1. A pattern evaluating apparatus comprising: a scanning electron microscope detecting a secondary electron or reflected electron by irradiation of an electron beam to a specimen formed with line pattern, and retrieving a two-dimensional distribution data of the secondary electron signal or the reflected electron signal; and a data processing apparatus for processing the two-dimensional distribution data; wherein the data processing apparatus retrieves a plurality of edge points data of the line pattern from the two-dimensional distribution data; generates data set of the edge points with different averaging parameter that defines a range of averaging to longitudinal direction of the line pattern, by calculating an averaging process to the data set within a range defined by the averaging parameter; calculating a standard deviation of the roughness of the data set corresponding to each averaging parameter; and calculating a noise-caused bias of the roughness included based on relationship between the standard deviation and the averaging parameter.