A linear actuator includes a guide mechanism and a retainer. On a guide block of the guide mechanism, a pair of ball-circulating grooves is formed on the lower surface that faces a cylinder body. Multiple balls are loaded in the ball-circulating grooves. Paired cover blocks are respectively mounted on the two ends of the guide block. The retainer has the form of paired ball clips, which are formed as arms to engage with the cover blocks when the clips are inserted in the ball-circulating grooves to hold the balls. Paired cover plates are respectively mounted on the end faces of the cover blocks, and arm-shaped cover clips are mounted so as to hold the cover plates. The cover plates, the cover blocks, and the guide block are thereby integrally linked.