PURPOSE: To display a TV broadcast on a screen when no intruder exists to arouse the attention of a viewer to the screen and then turning off the display on the screen when intruder exists to let the viewer know the presence of an intruder respectively, by performing the monitor with use of the optical signal converted from the TV signal.

CONSTITUTION: The TV signal received through an antenna 4 is converted into the optical signal through a converting part 2 and sent to a reconverting part 18. The part 2 is set at an edge part of a monitor section 16. The part 18 is set opposite to the part 2 with intervention of the monitor section 16. The part 18 converts again the optical signal given from the part 2 into the TV signal and supplies it to a TV picture receiver 30. This TV signal is converted into the DC voltage and compared with the reference voltage through a comparator 42. The comparator 42 actuates a buzzer 48 when the DC voltage is lower than the reference voltage. If an intruder enters the section 16, no optical signal is supplied to the part 18 to disappear the screen of the receiver 30. At the same time, the DC voltage becomes lower than the reference voltage. Thus the buzzer 48 works to inform the presence of an intruder.