An automatic drive mehanism for controlling a travelling speed of a vehicle at a preset value by transmitting the rotational movement of the members on the input shaft side driven by an electric motor to the output shaft side through an electromagnetic clutch to rotate an output member disposed on the output shaft side through a desired angle.The automatic drive mechanism comprises a stationary housing to be fixed to the vehicle, an electric motor, reduction gears, a coil cover driven by the reduction gears, an electromagnetic coil fixed to the housing and surrounded by the coil cover, a magnetic core member joined to the coil cover and rotating at a central portion of the electromagnetic coil, a clutch plate facing the coil cover, a friction engagement member intervening between the clutch plate and the coil cover, an output shaft joined to the clutch plate and supported rotatably and shiftably in the direction of its axis by the housing, an output member joined to the output shaft and associated with an accelerator mechanism of the vehicle, feed means arranged to extend through a first angular slot provided in the clutch plate and energizing the electromagnetic coil, and fixing members extending through a second angular slot provided in the clutch plate and fixing the electromagnetic coil to the housing.