A resonance-type contactless power supply adjusts a phase difference of an inverter control signal in a current cycle in a manner the same as that in a previous cycle in a case that a power parameter in the current cycle and that in the previous cycle satisfy a predetermined relationship, and adjusts the phase difference of the inverter control signal in the current cycle in a manner opposite to that in the previous cycle in a case that the power parameter in the current cycle and that in the previous cycle don't satisfy the predetermined relationship. The power parameter represents system efficiency. Thus, a suitable input current or voltage of the transmitter-side resonant circuit is determined by scanning actually, so that the system can operate at optimal efficiency.