4. 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 plus-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 plus range of a threshold value from the average to the edge segments along the center, and setting a line obtained by the linear approximation as an edge candidate.
4. 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 plus-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 plus range of a threshold value from the average to the edge segments along the center, and setting a line obtained by the linear approximation as an edge candidate. 6. The boundary detecting method according to claim 4 , wherein said candidate determining step further comprises 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 the edge segments along the center, and setting a line obtained by the linear approximation as an edge candidate.