PURPOSE: To determine a linear structure having approximate characteristic values by dividing each linear structure contained in an image into curve sections and displaying all sections having at least one characteristic value belonging to a predetermined same range.

CONSTITUTION: Measurements are determined based on parameters along two axes and composed along a linear structure to form an image where the linear structure is distributed preferentially along one axis and then the linear structure contained in the image is mapped. Each linear structure is divided into a plurality of curve sections touching each other at the ends thereof and a curve section is selected such that the maximum distance between a point in the curve section and a line connecting the opposite ends thereof does not exceed a given limit value. Each section is then allotted with one characteristic value characteristic of all measurements of parameters associated with the section and all sections having at least one characteristic value belonging to a predetermined same range are displayed.