It is an object of the present invention to provide a monitoring system that captures and inputs video images of a crowd with a surveillance camera for detecting normal and/or various abnormal states. The monitoring system (MS) comprises: a video image input means (1) that captures and inputs the video images of the crowd; an optical-flow calculating means (2) that calculates optical flows from the video images captured at different times; a judgment-block setting means (3) that sets judgment blocks on a surveillance area of each video image, each judgment block serving as a unit to be judged about an abnormality; an optical-flow attribute counting means (4) that counts an attribute of optical flows generated in each judgment block; a normality-evaluating-index calculating means (5) that calculates, from the counted attribute of the optical flows, an evaluation value of a normality-evaluating index for evaluating a normality of the judgment block; an abnormal-state judging means (6) that judges, from the evaluation value of the normality-evaluating index, the abnormal state of the judgment block; and an output means (7) that outputs a result of the judgment made by the abnormal-state judging means onto the video image.