An authentication apparatus performs local and global corrections on image data. Local correction uses the shape of a local line indicated by line information contained in a neighboring region on image data whose feature value extracted from the shape of a line is used for authentication. Global correction uses the shape of a global line indicated by line information contained in a region larger than the neighboring region. The authentication apparatus calculates the difference between line information contained in image data corrected by local correction and line information contained in image data corrected by global correction and compares the difference with a threshold. If the difference is less than the threshold, the authentication apparatus outputs, as line information contained in image data, line information corrected by local correction and, if the difference is greater, it outputs, as line information contained in image data, line information corrected by global correction.