To reduce unwanted radiation of electromagnetic waves that is radiated by a plurality of power supply devices into the space.

A power supply device supplies power to a power receiving device in non-contact manner. There are provided first communication means which, when an external power supply device supplies power to the power receiving device in non-contact manner, receives first data representing a phase of a first power signal for the external power supply device to supply power from the external power supply device, and comparison means which compares the first data with second data representing a phase of a second power signal for the power supply device to supply power to the power receiving device in non-contact manner. The phase of the second power signal is controlled according to a comparison result of the comparison means.