1. A system of detecting a stain, comprising: an image pickup unit that does not have a movable system and is fixed at one point for continuously picking up images; a picked-up image storage unit for storing images picked up by the image pickup unit; a subject region extraction unit for detecting that a target subject is in the picked-up image stored in the picked-up image storage unit, extracting a region of the subject, and generating a region extraction image; a region extraction image storage unit for accumulating a plurality of images which are recently picked up and which undergo region extraction performed by the subject region extraction unit; a stain level calculation unit for calculating a stain level representing a probability of the stain being present by comparing, on a pixel-by-pixel basis, the plurality of recent region extraction images accumulated in the region extraction image storage unit; a stain level storage unit for storing the calculated stain level for each pixel; and a stain determination unit for evaluating the stain level for each pixel read from the stain level storage unit, and determining whether or not the stain is present.