A hub unit for use in electrically assisted bicycles is improved in an effect to cool a motor incorporated in the unit. The hub unit has a motor 9 comprising a stator 3 and a rotor 4, and a hub 7 enclosing the motor 9 therein and rotatingly drivable by the rotation of the rotor 4. The unit has a pump 8 for discharging air from inside the hub 7 with the rotation of the hub 7. The air is discharged from inside the hub 7 by the operation of the pump 8, whereby air is admitted into the hub 7 from outside through an air port 59 formed in the hub 7 and is supplied to the motor 9 to cool the motor 9 and reliably prevent the motor from overheating.