A wireless energy transfer device, comprising: a power transmission unit that comprises a power transmission antenna adapted to generate an oscillating magnetic field or oscillating electric field; a power reception unit, which is provided with a receiving antenna, and which is adapted to convert the oscillating magnetic field or the oscillating electric field of the power transmission unit to electric power with the receiving antenna; and an electric power theft suppression unit that is adapted to suppress a theft of energy with the oscillating magnetic field or the oscillating electric field of the power transmission unit and that comprises: a frequency changing unit that is adapted to change a frequency of the oscillating magnetic field or the oscillating electric field generated by the power transmission unit; and a first resonance frequency changing unit that is adapted to change a resonance frequency of the receiving antenna according to the change in the frequency of the oscillating magnetic field or the oscillating electric field generated by the power transmission unit characterized by the electric power theft suppression unit further comprising a detection unit that is adapted to detect a second oscillating magnetic field or a second oscillating electric field different in amplitude or phase from the oscillating magnetic field or oscillating electric field generated by the power transmission unit during ordinary transfer of electric power that is not being attempted to be stolen and formed in the vicinity of the power transmission unit, the detection unit is installed at a location at a predetermined distance from the power transmission antenna; and an electric power theft determination unit that is adapted to determine whether the electric power is stolen based on a status of the second oscillating magnetic field or the second oscillating electric field detected by the detection unit during transmission of the electric power from the power transmission unit to the power reception unit. a power transmission unit that comprises a power transmission antenna adapted to generate an oscillating magnetic field or oscillating electric field; a power reception unit, which is provided with a receiving antenna, and which is adapted to convert the oscillating magnetic field or the oscillating electric field of the power transmission unit to electric power with the receiving antenna; and an electric power theft suppression unit that is adapted to suppress a theft of energy with the oscillating magnetic field or the oscillating electric field of the power transmission unit and that comprises: a frequency changing unit that is adapted to change a frequency of the oscillating magnetic field or the oscillating electric field generated by the power transmission unit; and a first resonance frequency changing unit that is adapted to change a resonance frequency of the receiving antenna according to the change in the frequency of the oscillating magnetic field or the oscillating electric field generated by the power transmission unit a detection unit that is adapted to detect a second oscillating magnetic field or a second oscillating electric field different in amplitude or phase from the oscillating magnetic field or oscillating electric field generated by the power transmission unit during ordinary transfer of electric power that is not being attempted to be stolen and formed in the vicinity of the power transmission unit, the detection unit is installed at a location at a predetermined distance from the power transmission antenna; and an electric power theft determination unit that is adapted to determine whether the electric power is stolen based on a status of the second oscillating magnetic field or the second oscillating electric field detected by the detection unit during transmission of the electric power from the power transmission unit to the power reception unit.