In a motor power transmission device (30), wherein rotational power of an electric motor (40) is input to a sun gear (51) of a planetary speed reducer (50), and then the rotation of the sun gear (51) is converted to rotation of a planetary carrier (55), and wherein the planetary carrier (55) rotatably supports planetary gears (61, 62) and converts revolving motion of the planetary gears (61, 62) into rotating motion, and the rotation of the planetary carrier (55) is then output to an output shaft (15), a torque limiter mechanism (57) is interposed between the planetary carrier (55) and the output shaft (15). Thus, even if excessive back torque is input to the output shaft, transmission of the back torque to the speed reducer is blocked to protect the speed reducer.