To improve the reliability by adding an auxiliary nonreversing function of fail safe, adding to a main nonreversing function which allows the rotation only in one direction, in a driving device such as an actuator connecting its load to a prime mover.

A linear ball screw type actuator having a bidirectional nonreversing device which has a main reversing property and an auxiliary nonreversing property commonly is provided. The auxiliary nonreversing property operates only following the loss of function of the main nonreversing property, and the friction and the fatigue of the composing member of an auxiliary nonreversing device is prevented so as to secure the maximum reliability. The auxiliary nonreversing device is provided by an auxiliary claw 64 supported at the front end of an eccentric arm 58. The angular position of the eccentric arm 58 is used to display the operating condition of this nonreverse device. The angular movement of the eccentric arm 58 is used to trigger the engagement of an auxiliary lock device when the main nonreversing property is out of order.