1. A hidden line processing method for avoiding projection of lines hidden by a plurality of polygons in projecting a three-dimensional model consisting of the polygons onto a two-dimensional plane, comprising the steps of: obtaining the maximum value PZ max of the Z-axis direction component of each vertex in a viewpoint coordinate system for each of the plurality of polygons belonging to parts constituting the three-dimensional model; sorting the plurality of polygons in a descending order based on the obtained maximum values PZ max ; obtaining a value LZ min that is the smaller of the Z-axis direction components in the viewpoint coordinate system of two (2) end points of an arbitrary line obtained from the plurality of polygons; and comparing the maximum value PZ max of the Z-axis direction component of the plurality of polygons with the value LZ min that is the smaller of the above obtained Z-axis direction components of the arbitrary line, in the sorted order, wherein at the time when LZ min &gE;PZ max, determination of whether or not the lines are hidden lines is avoided for polygons subsequent to the plurality of polygons sorted.