a linear actuator is provided which includes a motor and a screw as an output shaft and in which the rotational movement of the motor is converted into the linear movement of the screw , wherein at least one retainer to hold a plurality of balls is disposed at the inner periphery of a hollow rotor of the motor , a ball screw is formed at the outer periphery of the screw , the balls are engaged with the ball screw such that the screw is set coaxial to the rotor , and wherein the screw provided with the ball screw is moved linearly by means of the rotation of the balls disposed circumferentially at the inner periphery of the rotor .