A sweat gland pore extraction means 17 radially comparatively evaluates densities of near pixel groups around an intended pixel by use of a predetermined radial mask to determine whether to encounter pixels which can be regarded as ridges in every direction, records the direction which does not encounter the ridge pixels as an open direction, and extracts the inside of a pixel group which can be regarded as ridge pixels as a sweat gland pore pixel group. 汗腺口抽出手段17は、予め定められた放射状マスクを用いて注目画素を中心とする近傍画素群の濃度を放射状に比較評価し、各々の方向に対して隆線と見なせる画素にぶつかるか否かを判定し、隆線画素にぶつからない方向をオープン方向として記録するとともに、隆線画素と見なせる画素群の内側を汗腺口画素群として抽出する。 - 特許庁