A linear actuator includes: a pinion shaft coaxially coupled to a motor shaft; a driven gear that is engaged with a pinion gear on the pinion shaft; a nut that rotates with the driven gear; a feed screw mechanism that converts rotation movement of the nut into linear movement of a rod; a coupling part allowing an outer peripheral surface of the motor shaft to be serration coupled to an inner peripheral surface of a vertical hole in the pinion shaft; and a pressurizing unit that generates a pressure that axially acts between e motor shaft and the pinion shaft in the coupling part.