PURPOSE: To cause the warning display on a sensor unit side to be ceased when an informing state is set, by providing a means which selectively sets the informing state for informing of a visitor and another state for informing an abnormal case.

CONSTITUTION: A means which switches the direct current to be made to flow to a circuit to a pulse-like current of such a duty that the warning lamp provided in a sensor unit cannot light at the time of informing state, is installed to a crime preventing unit. Since an input 'H' is inputted to one terminal of an AND gate 12 when the informing state is set, a signal synchronized with the transmitting signal of a transmitter 9 is outputted from the AND gate 12 and a transistor Tr2 intermittently operates and, as a result, the pulse-like current flows to the circuit 3. When the duty of the pulse-like current is set to such a degree that at which a warning lamp L2 composed of LEDs does not light, the warning lamp L2 does not light when the information state is set.