PROBLEM TO BE SOLVED: To miniaturize a device by shortening the overall length of an actuator while maintaining the function of the actuator.SOLUTION: A linear actuator 1 includes: a shaft 26 that is forward/reverse rotated by a motor via a worm 36 and a worm wheel 41; a screw nut 29 threadably mounted to the shaft 26; and a piston tube 10 that is fixed to the screw nut 29 and that advances and retreats in accordance with the rotation of the shaft 26. A metal coupling 42 is attached on the radial-direction inside of the worm wheel 41. A one-way clutch 44 is attached on the radial-direction inside of the coupling 42. A brake unit 45 that does not apply braking force during forward rotation, during which the piston tube 10 extends, and applies braking force during reverse rotation of the piston tube 10 is connected to the one-way clutch 44.