Disclosed herein are a decelerator in which a first deceleration unit of a planet gear type is provided inside a second deceleration unit of a cycloid type so as to apply eccentric rotation force to the second deceleration unit, and a motor brake with the same. The decelerator restricts an increase in the axial thickness of a motor and achieves an effectively increased deceleration ratio, and the motor brake equipped with the decelerator restricts an increase of the overall length thereof due to the decelerator, achieving enhanced space utility.