A parking aid system 1 includes: a plurality of power supply devices installed at every parking space, and a parking aid device 5 which is built into a vehicle 2. The parking aid device 5 includes an on-board communication device 21 which acquires a first positional information sent from a power supply device, and an alignment aid unit 23 which aids in alignment to a power supply unit of the power supply device based on the first positional information. The alignment aid unit 23 calculates a path of the vehicle 2, specifies a power supply unit on the path of the vehicle 2from among the power supply units of the plurality of power supply devices based on the first positional information, and executes authentication which permits transmission of electrical power with the power supply device of this specified power supply unit.