To provide a hub motor unit of a simple structure for sufficiently cooling a motor.

A motor 30 is provided on an axle of wheels on the same axis as the axle on an inner side of a rotatably supported hub 10. In the hub 10, hub end plates 11a and 11b fixed to a frame member are supported on both sides, and a cylindrical hub housing 17 is rotatably supported by the axle therebetween. In the motor 30, a cylindrical outer circumferential housing 35 as a motor housing 31, the hub end plate 11a to close one opening thereof, and a motor lid body 36 to close the other opening of the outer circumferential housing 35 are provided. A rotor 34 is rotatably supported by the motor housing 31, and the rotational force of the rotor 34 is transmitted to the cylindrical hub housing 17 to rotate the wheels.