A linear actuator is provided with an outer tube and an inner tube inserted in the outer tube so as to be free to slide. The linear actuator is also provided with a rod erected at a central axis part of the inner tube and forming an annular space between the rod and the inner tube, a rod guide which slides along an inner surface of the outer tube and guides the rod along an axial direction, a plurality of permanent magnets arranged along the axial direction in the rod, and a plurality of coils retained by the outer tube so as to face the permanent magnets. A cooling fluid is filled in an internal space formed between the outer tube and the inner tube.