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 monitor terminal, wherein the database includes a specified person list and an unspecified person list, wherein the specified person detection server updates the database by acquiring an image captured by the image pickup device, calculating a degree of reliability for identifying a person in the acquired image by using an image feature quantity 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, and 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 the monitor terminal.