Apparatus and method for controlling resonator of wireless power transfer system

Disclosed are an apparatus and a method for changing a size of a magnetic field in which a resonator outputs or receives. A source device for transmitting a magnetic field by resonating with a target device includes a source resonator including a plurality of loop circuits for generating a different magnetic field according to a length; and a circuit selector for selecting any one loop circuit among the loop circuits depending on the information related to the target device.