The invention relates to a power supply apparatus, a power receiving apparatus, a power supply system and a method of controlling power supply. The power supply apparatus includes: a power supply unitconfigured to supply power to a power receiving apparatus through a magnetic field; a measurement unit configured to measure an electrical characteristic value and generate a measurement value; a power receiving unit configured to provide a set value; and a foreign object detection unit configured to detect a foreign object in the magnetic field based on the set value and the measured value.