A hub unit having a sensor comprises a hub unit 1 having a wheel-side raceway member 4, a body-side raceway member 5 and two rows of rolling bodies 6, and a sensor device 2 provided on the hub unit 1. The body-side raceway member 5 has a cylindrical portion 16 and a flange portion 17 provided with an insertion hole for a bolt for fastening the hub unit 1 to a vehicle body. The sensor device 2 has a magnetic sensor 31 supported by a support member 33 fixed to the flange portion 17 and serving as a displacement sensor for detecting variations in the distance between the sensor and the cylindrical portion 16. A magnetized portion 34 is provided on the cylindrical portion 16 at a location opposed to the sensor.