Surveillance system and surveillance method for detecting intruders. An image-pickup unit takes an image of a predetermined area, a person detection unit detects, on the basis of the taken image, information concerning people pictured in the image, radio terminal units are held and carried by people, radio reception units receive signals transmitted by radio from the radio terminal units present in the predetermined area, a terminal detection unit detects information concerning the radio terminal units on the basis of the received signals and an intruder decision unit detects information concerning an intruder on the basis of the information concerning the detected persons and the information detected in connection with the radio terminal units.