An in-wheel motor system comprising: an in-wheel motor for mounting in a wheel of a vehicle, the vehicle having a vehicle body; a power transmitter configured to utilize a magnetic resonance coupling method, for transmitting power wirelessly from the vehicle body to the in-wheel motor, the power transmitter comprising a wheel side coil configured to receive wirelessly transmitted power at the wheel side during powering; a communication interface configured to communicate between the vehicle body and the wheel; a wheel side conversion unit configured to convert a first AC voltage (Val) from the wheel side coil into a first DC voltage during powering; a wheel side capacitor configured to smooth the first DC voltage during powering; a motor conversion unit configured to convert the first DC voltage into a motor AC voltage for driving the in-wheel motor during powering; a wheel side controller configured to control, with a first control signal, switching of a switch provided in the wheel side conversion unit; a battery; and a vehicle body side conversion unit; wherein the vehicle body side conversion unit is configured to convert a second DC voltage from the battery into a second AC voltage and to output the second AC voltage to a vehicle body side coil during powering; wherein the power transmitter further comprises a wheel side resonance capacitor inserted in series between the wheel side coil and the wheel side conversion unit; wherein the communication interface is configured to wirelessly transmit a control signal for driving the in-wheel motor; wherein the wheel side controller is configured to generate the first control signal with the timing at a zero cross point of a current of the wheel side coil as a reference, and the in-wheel motor system further comprises a vehicle body side controller configured to control, with a second control signal, switching of a switch provided in the vehicle body side conversion unit in accordance with a control signal from the communication interface; wherein the vehicle body side controller is configured to generate the second control signal so as to control the first DC voltage to be constant. an in-wheel motor for mounting in a wheel of a vehicle, the vehicle having a vehicle body; a power transmitter configured to utilize a magnetic resonance coupling method, for transmitting power wirelessly from the vehicle body to the in-wheel motor, the power transmitter comprising a wheel side coil configured to receive wirelessly transmitted power at the wheel side during powering; a communication interface configured to communicate between the vehicle body and the wheel; a wheel side conversion unit configured to convert a first AC voltage (Val) from the wheel side coil into a first DC voltage during powering; a wheel side capacitor configured to smooth the first DC voltage during powering; a motor conversion unit configured to convert the first DC voltage into a motor AC voltage for driving the in-wheel motor during powering; a wheel side controller configured to control, with a first control signal, switching of a switch provided in the wheel side conversion unit; a battery; and a vehicle body side conversion unit; wherein the vehicle body side conversion unit is configured to convert a second DC voltage from the battery into a second AC voltage and to output the second AC voltage to a vehicle body side coil during powering; wherein the power transmitter further comprises a wheel side resonance capacitor inserted in series between the wheel side coil and the wheel side conversion unit; wherein the communication interface is configured to wirelessly transmit a control signal for driving the in-wheel motor; wherein the wheel side controller is configured to generate the first control signal with the timing at a zero cross point of a current of the wheel side coil as a reference, and the in-wheel motor system further comprises a vehicle body side controller configured to control, with a second control signal, switching of a switch provided in the vehicle body side conversion unit in accordance with a control signal from the communication interface; wherein the vehicle body side controller is configured to generate the second control signal so as to control the first DC voltage to be constant.