Abstract: A linear actuator includes two axially opposed bearings with built-in one-way clutch assembled in a housing member with respective outer races secured to the housing member such that respective directions in which inner races of the bearings are locked against rotation are opposed to each other, a ball bearing shaft relatively and rotatably supported on the inner races of the bearings with bearing bushings disposed between the ball screw shaft and the respective bearings, and two load transmitting collars fixed on the ball screw shaft. When the ball screw shaft is subjected to a thrust load applied from a ball nut in a direction to retract the ball screw shaft, one of the inner race is locked against rotation to thereby exert a braking torque to the corresponding load transmitting collar via an associated friction collar.