A charger incorporated in a vehicle wheel supporting rolling bearing unit detects an output voltage of an electric generator and calculates a rotation speed of a hub. When a rotation speed of the hub is in a low speed state where a sensor and a wireless communication device are operated by electric power supplied 5 by a battery, a frequency of wireless communication performed between the wireless communication device and a calculator disposed on a vehicle body side is changed according to the rotation speed of the hub or a traveling speed of a vehicle.