Abstract: A worm gear for driving devices such as automobile power windows and power seats the operating positions of which have to be precisely determined. The worm gear motor has a built-in sensor means having a torque transmission plate engaged with a motor-driven worm wheel having an output shaft in its center and at least one circumferentially extending notch on the periphery, a magnet disposed on one side of the torque transmission plate along the path of the notch during rotation of the torque transmission plate and a magnetic sensor disposed on the other side of the torque transmission plate opposite the magnet. The sensor repeats ON-OFF cycles according to the presence or absence of the notch while said motor is in operation, detecting a predetermined rotational position of the output shaft by the signal attained by the sensor, and attaining a high positional determination of the motor output shaft regardless of backlash of engaged gears and mechanical inertia of the motor components.