5. The boundary detecting method according to claim 3 , wherein said candidate determining step further comprises a step of obtaining an average of absolute values of a difference between adjacent edges in a search direction, and a minus-side step of making a linear approximation for a set obtained by adding an edge segment whose absolute value of a difference is within a minus range of a threshold value from the average to edge segments along the center, and setting a line obtained by the linear approximation as an edge candidate.
3. The boundary detecting method according to claim 1 , wherein said candidate determining step further comprises a step of making a linear approximation for a part of the edge segments along a center among the edge segments for all of the search lines to find an edge candidate.