A moisture intrusion detection system includes: an industrial robot that has a closed space defined thereinside; an air supply mechanism that supplies compressed air to the closed space; an air vent passage that is provided in the industrial robot to establish communication between an external space outside the industrial robot and the closed space, and guides, to the external space, air ejected from the closed space upon supply of the compressed air; a humidity sensor that detects a humidity of ejected air that is ejected from the air vent passage to the external space upon supply of the compressed air; and a controller that determines presence or absence of moisture intruding into the closed space based on a detected humidity of the ejected air, the detected humidity being detected by the humidity sensor.