Disclosed is an automatic door actuator which comprises arithmetic unit for calculating a weight of a door, a distance of movement of the door and braking points of the door through learn control of door actuation which is effected once every time a power supply switch is turned on and irrespective of the on-off condition of a normal door actuation start switch; first memory for storing results of calculation by the arithmetic unit; second memory for previously storing various phase angles for starting operation, for deceleration braking operation and for predetermined low velocity control, corresponding to various weights of the door; whereby the control unit is responsive to the first memory to read out from the second memory the phase angles for starting operation, for deceleration braking operation and for predetermined low velocity control corresponding to the actual weight of the door determined in the learn control to thereby supply a trigger signal to semiconductor controlled rectifying elements in accordance with the read-out phase angles corresponding to the actual door weight and the distance of movement of the door and the braking points of the door determined in the learn control.
