A linear actuator includes a cylinder main body, which is provided at one end thereof with a lock mechanism capable of restricting displacement of a slide table. The lock mechanism is equipped with a lock plate and a sub-piston. The lock plate is rotatable toward a side of the slide table by an elastic force of a spring, inserted into an insertion groove, and restricts displacement of the slide table. The sub-piston is displaced by a pressure fluid supplied to a supply port, and releases a displacement restricted state of the slide table by the lock plate.