To provide a motor with built-in speed reducing mechanism which can improve the centering accuracy of a sun roller, planet roller, and outer ring.

The rear end section of the rotating shaft 7 of a rotating body (core) 8 positioned in a motor case 4 is supported by means of the case 4 through a rear bearing 26 and the planet roller 15 of a planet roller type speed reducing mechanism 3 is brought into contact with the front end section of the shaft 7 so that the roller 15 can roll on the peripheral surface of the shaft 7. In addition, an output shaft section l7a which is made coaxial to the shaft 7 and extended outward is integrally formed with a carrier 17 which supports the roller 15 and the output shaft section 17a is supported by means of the case 4 through a front bearing 24.