A door driving apparatus having a motor, a sleeve formed at a drive end of a rotor of the motor, a pinion integrally fixed to a rotation shaft integrated with a spline shaft detachably fitted to the sleeve, and left and right racks disposed opposite to each other with respect to the pinion and engaged with the pinion so that the left and right racks can move in directions substantially parallel and opposite to each other. When the motor is individually removed from a rack and pinion casing, the pinion is left in the pinion and rack casing while upper and lower sides of the pinion are engaged with the right and left racks, respectively. Accordingly, the relative positions of the left and right racks are unchanged.