1. An intrusion detection device comprising: a light emitting unit which forms a plurality of light emitting marks arranged on a boundary of an intrusion monitoring area; an imaging device which images an area including the plurality of light emitting marks, and outputs the captured image; and a detection unit which detects predetermined marks which are different from the light emitting marks, from the output image, sets a masking area on the image based on the detected marks, and detects an intrusion material in the intrusion monitoring area based on the image in an area other than the masking area.