1. A hardness tester for determining hardness of a sample by measuring a size of an impression formed on a surface of the sample, placed on a sample stage, by impressing an indenter with predetermined test load, comprising: an image capture controller for obtaining an image data of the surface of the sample by controlling an image capturer and making the image capturer capture an image of the surface; an impression region extractor for extracting an impression region formed on the surface of the sample based on the image data obtained by the image capture controller; an impression vertex extractor for extracting a vertex for measuring the impression to measure a size of the impression based on the impression region extracted by the impression region extractor; and a hardness calculator for calculating hardness of the sample based on the vertexes for measuring the impression extracted by the impression vertex extractor; wherein the impression region extractor binarizes the image data of the surface of the sample, applies a reduction processing and an expansion processing to the binarized image data, applies a distance conversion processing to the reduction/expansion processed image data, and extracts a closed region corresponding to a contour of the indenter by using the distance-converted image data, and upon unsuccessful extraction of the closed region corresponding to the contour of the indenter, changes a level of binarization and thereafter re-extracts the impression region formed on the surface of the sample, and the impression vertex extractor estimates the vertex for measuring the impression based on a profile of the closed region extracted by the impression region extractor, and extracts a point in the binarized image data before the reduction processing and the expansion processing, as the vertex for measuring the impression, that agrees with a predetermined condition based on the estimated vertex.