A linear motion actuator includes an outer ring member mounted in a housing, a rotary shaft rotated by a motor, planetary rollers mounted between the outer ring member and the shaft and supported by a carrier rotatable about the shaft. A helical rib is formed on the inner surface of the outer ring member which is engaged in helical grooves formed in outer surfaces of the planetary rollers. When the shaft rotates, the planetary rollers rotate and revolve, thereby axially moving the outer ring member. Uneven axial loads applied to the planetary rollers from the outer ring member are supported by thrust bearings disposed between the planetary rollers and an inner disk of the carrier. Aligning seats are provided between each opposed pair of the planetary rollers and the thrust bearings such that the contact surfaces of the aligning seats shift relative to each other under the uneven axial loads.