Abstract: A power plug, power outlet, power supply device and power supply system for preventing electricity theft with relatively simple configuration are provided. A power outlet unit supplies power to a power plug unit to establish communication over a power line between a PLC block of the power outlet unit and a PLC block of the power plug unit. Upon transmission of an ID number from the power plug unit, the transmitted ID number is compared with an ID number stored in a memory. If the ID number matches the ID number in the memory, power is continuously supplied to the power plug unit. If the ID number does not match, the power supply is shut down to prevent electricity theft.