PURPOSE: To get such a habit that a dangerous apparatus is unplugged from a socket securely by turning on and off a display through a radio transmitter under the control of a switch which operates associatively with the insertion and extraction of plug blades into and from the socket.

CONSTITUTION: When plug blades 1 and inserted into the socket, the blades connecting with a load through a plug 4 are connected to the blades 1 through electric wires 3. The insertion force in this insertion moves a blade block 5 in a space 7, and the switch 10 is associated with the movement to supply the electric power from a battery 13 to the ratio transmitter 14. A transmit radio wave is received by a receiver to turn on the lamp of a display device, and even after the block 5 returns to a neutral position on the insertion of the blades 1, the lighting state is held. When the blades 1 are extracted in the said state, the block 5 moves in a space 9 and the lamp of the display lamp is turned off. Thus the monitor display is made by the inserting and extracting the blades 1 to form the habit that a dangerous apparatus such as an electric heater is extracted securely from a socket, improving the safety.