A groove is provided to the rear of an inner wall of a cylinder. A locking ram capable of freely sliding in a radial direction and a cam rod having a profile changing with a transition from the rear to the front from a small diameter part to a large diameter part that are provided relative to a piston. The inner end of the locking ram makes contact with the profile, and the cam rod is urged to the rear by the spring, so that the cam rod can be pushed to the front by a push rod. The piston can therefore be locked with a simple structure, and it is possible to provide a linear actuator where locking can be released with a straightforward operation even when a pressurized oil source fails.