A linear actuator includes a guide block constituting a guide mechanism. In the guide block, a pair of installation grooves are formed in a lower surface facing toward a cylinder main body. Ball circulation members having therein ball circulation holes are installed respectively in the installation grooves. Additionally, ball circulation passages through which balls circulate are provided. The ball circulation passages are made up from roll-reversing sections disposed on opposite ends of the ball circulation members, the ball circulation holes, second ball guide grooves formed in both side surfaces of the guide block, and first ball guide grooves of the slide table.