With a metal foreign object detecting method, a metal detection circuit arranged in a contactless power supply device transmits an oscillation signal to a modulation circuit of an electric appliance. The modulation circuit generates a square wave pulse signal from the oscillation signal. The cycle of the oscillation signal received by the modulation circuit changes in accordance with whether or not a metal piece is present. The modulation circuit modulates the square wave pulse signal to generate a modulated wave signal and transmits the modulated wave signal to the metal detection circuit of the power supply device. The metal detection circuit demodulates the modulated wave signal and determines whether or not a metal piece is present based on the cycle of the demodulated signal corresponding to the square wave pulse signal.