a driving part which generates a rotation force;

a first gear part which transmits the rotation force of the driving part;

a second gear part including a rotation shaft which receives the rotation force from the first gear part to rotate; and

a rotation shaft control part which is disposed at one side of the second gear part on the rotation shaft, provides a degree of rotational freedom to the rotation shaft during braking, and prevents reverse rotation of the second gear part due to a reverse rotation torque by fixing the rotation shaft when the braking is completed,

wherein the second gear part further includes:

an inner gear coupled to the rotation shaft; and

an outer gear having a ring shape of which an inner circumferential surface is coupled to the inner gear and in which gear teeth engaged with the first gear part are formed on an outer circumferential surface of the outer gear,

wherein the inner gear includes:

a small diameter part which is coupled to the rotation shaft and in which gear teeth are provided on an outer circumferential surface of the small diameter part; and

a large diameter part which is expanded by communicating with the small diameter part and in which gear teeth are provided on an inner circumferential surface of the large diameter part.