To provide a bearing unit for a driving wheel which can measure the rational speed of the driving wheel at high accuracy, not affected by relative tilting between a stationary ring and a rotary ring.

The bearing unit includes the stationary ring (outer ring)2 fixed to the car body side and ordinarily maintained in the non-rotational state, the rotary ring (inner ring)4 for transmitting the driving force of a drive unit to the wheel to rotate it, rolling members 6 and 8 incorporated between the stationary ring and the rotary ring, and a rotation detecting mechanism for detecting the rotational speed of the wheel. A constant velocity joint (CVJ) is connected to the rotary ring. The rotation detecting mechanism is provided with an encoder 36 rotating together with the rotary ring, and a sensor 38 for detecting an information characteristic recorded in the encoder. The sensor is supported by a sensor support 40 in opposition to the encoder. The sensor support is connected to the rotary ring via a bearing assembly, and is ordinarily maintained in the non-rotational state by a rotation locking structure formed between the sensor support and the stationary ring.