1. An intrusion detection device that is configured to execute processes by a processor, the intrusion detection device comprising: a light emitting unit which forms a plurality of light emitting marks arranged on a boundary area between a safety area and an intrusion monitoring area; an imaging device configured to take an image of the boundary area including the plurality of light emitting marks, the imaging device configured to output the image in first through fourth states, wherein the first state corresponds to a reference image of the boundary area where no object exists, the second state corresponds to an intrusion image of the boundary area where a first object exists, the first object having no exception mark, the third state corresponds to an exceptional intrusion image of the boundary area where a second object exists, the second object having the exception mark, and the fourth state corresponds to a mixed image of the boundary area where the first and second objects exist; and a detector configured to detect the exception mark within the image in the third and fourth states, the detector configured to set a masking area corresponding to a shape of the second object existing in the boundary area based on a location of the exception mark, wherein the detector is configured to detect whether the first object is located outside the masking area in the fourth state, and the detector is configured to detect whether the first object is located in any part of the boundary area in the second state.