An abnormality behind a shielding object is discovered. There is provided a monitoring system including a video acquirer, a detector, and a notifier. The video acquirer of the monitoring system acquires a video. The detector of the monitoring system detects entering of a target object into a blind spot generated by a shielding object in the video and appearance of the target object from the blind spot. If the target object does not appear from the blind spot even after a first time elapses since entering of the target object into the blind spot, the notifier of the monitoring system makes a notification.