1. A person detection system comprising: an image pickup device for capturing images; a specified person detection server; a database storing personal information; and a plurality of monitor terminals, wherein the database includes a specified person list and an unspecified person list, wherein the specified person detection server updates the database by iteratively acquiring an image captured by the image pickup device, calculating a degree of reliability for identifying a person in the acquired image by using image feature quantities extracted from the acquired image, and storing image data of the acquired image on the unspecified person list based on the calculated degree of reliability, wherein the specified person detection server additionally registers, on the specified person list, the image captured by the image pickup device or the personal information of a person retrieved from the unspecified person list in response to a request from one of the monitor terminals, wherein the one of the monitor terminals comprises at least one of an image display unit, a detected object indicator, a search request generating unit, wherein another one of the monitor terminals comprises an additional registration operation unit, and wherein the image is additionally registered during the system operation based on a difference of the face image of the person caused by the shooting angle and variations of the face image caused by luminous source position, makeup, age-related change in face, or accessories.