A current frame portion (21) stores therein the most recent information of an image of those fingerprint images input from an inputted unit (1), while a previous frame portion (22) stores therein image information saved in the current frame portion (21). A pixel comparing portion (31) compares mutually corresponding pixel gray-level value information saved in the current frame portion (21) and the previous frame portion (22) respectively. A gray-level-value-changed pixels counting portion (32) counts the gray-level-value-increased pixels and the gray-level-value-decreased pixels based on the resultant information of that comparison of the pixel gray-level values. A pick-up timing deciding portion (33) utilizes the results obtained by the gray-level-value-changed pixels counting portion (32), to automatically detect a fingerprint image appropriate for matching in order to thus decide pick-up timing and also in order to output to an output unit (4) an appropriate fingerprint image based on the decision results.