A hub unit includes a double-row outer ring having a first inner peripheral raceway and a second inner peripheral raceway, a hub having a wheel-mounting flange provided with a reference surface for mounting the wheel formed on a vehicle outer end-side thereof, a first outer peripheral raceway opposed to the first inner peripheral raceway of the outer ring, a shaft body and an inner ring element having a second outer peripheral raceway opposed to the second inner peripheral raceway of the outer ring, rolling members interposed between the double-row inner peripheral raceway of the outer ring and the double-row outer peripheral raceway of the hub and cages. A rust-preventive layer is provided at least on the wheel-mounting reference surface of the flange.