In an electric linear motion actuator, it is proposed to uniformly distribute external thrust loads to respective planetary rollers while the planetary rollers are revolving around a rotary shaft while rotating about their respective axes with a helical rib of an outer ring member in engagement with circumferential grooves of the respective planetary rollers. The planetary rollers have bearing support surfaces on which thrust bearings are supported, respectively. The distance from the bearing support surface of each planetary roller to a predetermined reference position of each circumferential groove (6a) of the planetary roller is different from the corresponding distances for the other planetary rollers, whereby with the planetary rollers supported by the respective thrust bearings, the axial position of each circumferential groove coincides with the axial position of the portion of the helical rib of the outer ring member that is engaged in the circumferential groove.